Manual 21171759

Manual 21171759
Algorithmic Aspects of Machine Learning
Ankur Moitra
c Draft date March 30, 2014
Contents
Contents
i
Preface
1
1 Introduction
3
2 Nonnegative Matrix Factorization
5
2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2
Algebraic Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3
Stability and Separability . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4
Topic Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3 Tensor Methods
5
25
3.1
Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2
Perturbation Bounds . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.3
Phylogenetic Trees and HMMs . . . . . . . . . . . . . . . . . . . . . . 34
3.4
Community Detection . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.5
Extensions to Mixed Models . . . . . . . . . . . . . . . . . . . . . . . 44
3.6
Independent Component Analysis . . . . . . . . . . . . . . . . . . . . 50
4 Sparse Recovery
53
4.1
Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2
Uniqueness and Uncertainty Principles . . . . . . . . . . . . . . . . . 56
4.3
Pursuit Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
i
ii
CONTENTS
4.4
Prony’s Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.5
Compressed Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5 Dictionary Learning
71
5.1
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.2
Full Rank Dictionaries . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.3
Overcomplete Dictionaries . . . . . . . . . . . . . . . . . . . . . . . . 77
6 Gaussian Mixture Models
83
6.1
History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
6.2
Clustering-Based Algorithms . . . . . . . . . . . . . . . . . . . . . . . 86
6.3
Discussion of Density Estimation . . . . . . . . . . . . . . . . . . . . 89
6.4
Clustering-Free Algorithms . . . . . . . . . . . . . . . . . . . . . . . . 91
6.5
A Univariate Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.6
A View from Algebraic Geometry . . . . . . . . . . . . . . . . . . . . 101
7 Matrix Completion
105
7.1
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
7.2
Nuclear Norm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.3
Quantum Golfing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Bibliography
115
Preface
The monograph is based on the class “18.S996: Algorithmic Aspects of Machine
Learning” taught at MIT in Fall 2013. Thanks to the scribes Adam Hesterberg,
Adrian Vladu, Matt Coudron, Jan-Christian Hütter, Henry Yuen, Yufei Zhao, Hilary Finucane, Matthew Johnson, Kayhan Batmanghelich, Gautam Kamath, George
Chen, Pratiksha Thaker, Mohammad Bavarian, Vlad Firoiu, Madalina Persu, Cameron
Musco, Christopher Musco, Jing Lin, Timothy Chu, Yin-Tat Lee, Josh Alman,
Nathan Pinsker and Adam Bouland.
1
2
CONTENTS
Chapter 1
Introduction
This course will be organized around algorithmic issues that arise in machine learning. The usual paradigm for algorithm design is to give an algorithm that succeeds on
all possible inputs, but the difficulty is that almost all of the optimization problems
that arise in modern machine learning are computationally intractable. Nevertheless,
practitioners use a wide variety of heuristics that are successful in practice. However
we often do not understand when and why these approaches work (an issue we would
not have if our algorithms came with provable guarantees). The central questions
in this course are:
Question 1 Which models in machine learning lead to tractable algorithmic problems?
Worst-case analysis is comfortable because if an algorithm works in this model,
it certainly works in practice. But the optimization problems that machine learning
systems “solve” everyday are indeed hard in the worst-case. However these lower
bounds are not so frightening; many of the hard instances of machine learning
problems are not ones we would want to solve in practice anyways! We will see a
number of examples where choosing the right model will lead us to discover new
algorithms with provable guarantees, where we really can understand when and
why they work. In some cases, we will even be able to analyze approaches that
practitioners already use and give new insights into their behavior.
Question 2 Can new models – that better represent the instances we actually want
to solve in practice – be the inspiration for developing fundamentally new algorithms
for machine learning problems? Can we understand when and why widely used
heuristics work?
3
4
CHAPTER 1. INTRODUCTION
This course will focus on
(a) nonnegative matrix factorization
(b) topic modeling
(c) tensor decompositions
(d) sparse recovery
(e) dictionary learning
(f) learning mixtures models
(g) matrix completion
Hopefully more sections will be added to this course over time, since there are a
vast number of topics at the intersection of algorithms and machine learning left to
explore.
Chapter 2
Nonnegative Matrix Factorization
In this chapter we will explore the nonnegative matrix factorization problem. We
will first recap the motivations from this problem. Next, we give new algorithms
that we apply to the classic problem of learning the parameters of a topic model.
2.1
Introduction
In order to understand why nonnegative matrix factorization is useful in applications, it will be helpful to compare it to the singular value decomposition. We will
focus on applications of both of these to text analysis in this chapter.
Singular Value Decomposition
Given an m × n matrix M , its singular value decomposition is
M = U ΣV T
where U and V are orthonormal and Σ is diagonal and its entries are nonnegative.
Alternatively we can write
r
X
M=
ui σi viT
i=1
th
where ui is the i
entry of Σ.
column of U , vi is the ith column of V and σi is the ith diagonal
Every matrix has a singular value decomposition! In fact, this representation
can be quite useful in understanding the behavior of a linear operator or in general
5
6
CHAPTER 2. NONNEGATIVE MATRIX FACTORIZATION
for extracting significant “features” from a large data matrix. We will focus our
discussion of the singular value decomposition on the latter. One of the many useful
properties of this decomposition is that we can immediately read-off the best low
rank approximation to M from it.
Definition 2.1.1 The Frobenius norm of a matrix M is kM kF =
pP
P
ternately, if M = ri=1 ui σi viT , kM kF =
σi2 .
qP
i,j
2
Mi,j
. Al-
Consider the following optimization problem: Let B be the best rank k approximation to M in the Frobenius norm - i.e. B is the minimizer of kM − BkF
over all rank at most k matrices. Then we can without loss of generality choose B
to be the first k terms of the singular value decomposition.
Theorem 2.1.2 (Eckart-Young) The best rank k approximation to q
M in FrobePr
Pk
2
nius norm is attained by B = i=1 ui σi viT , and its error is kM −BkF =
i=k+1 σi .
This is one of the reasons why the singular value decomposition is so widely
useful: if we are given data in the form of a matrix M but we believe that the data
is approximately low-rank, a natural approach to making use of this structure is to
instead work with the best rank k approximation to M . This theorem is quite robust
and holds even when we change how we measure how good B is as an approximation
to M :
Definition 2.1.3
P The operator norm of a matrix M is kM k2 = max|v|=1 kM vk2 .
Then if M = ri=1 ui σi viT , ||M ||2 = σ1 (the largest singular value).
The
to M in the operator norm is also attained by B =
Pk best approximation
T
i=1 ui σi vi , in which case the error is kM − Bk2 = σk+1 .
Let us give one more interpretation of the singular value decomposition. We
can regard an m × n matrix M as a collection of n data points in <m . We associate
a distribution ∆ with this set of points which chooses a point uniformly at random.
Further suppose that the expectation of this distribution is zero. Our data is in
high dimension, and a natural question to ask is: how should we project our data
onto a one dimensional subspace in a manner that preserves as much information
as possible? One concrete goal is to find a direction u so that projecting ∆ on u
maximizes the variance (among all one-dimensional projections). The question leads
to another characterization of the singular vectors:
2.1. INTRODUCTION
7
kuT M k2
u1 = argmax
kuk2
and the maximum is σ1 . Similarly if we want to project onto a two-dimensional
subspace so as to maximize the projected variance we should project on span(u1 , u2 ).
Relatedly
kuT M k2
u2 = minu1 argmaxu⊥u1
kuk2
and the maximum is σ2 . This is called the variational characterization of singular
vectors. (Here we have assumed the singular values are distinct).
There are efficient algorithms to compute the singular value decomposition. If
n 6= m then these algorithms run in time O(mn2 ). The basic idea is to reduce M
to bidiagonal form using Householder reflections, and then to compute the singular
value decomposition from this representation using the QR algorithm. Next we will
describe an application to text analysis.
Applications to Text Analysis
Latent Semantic Indexing: [49]
Suppose we are give a large collection of documents, and we would like to
extract some hidden structure in this collection (either with the goal of performing
information retrieval, or clustering). The usual first step is to collect the data in a
very large, very sparse matrix:
Definition 2.1.4 The term-by-document matrix M is an m × n matrix where each
row represents a word, each column represents a document and the entry in row i,
column j is the number of times that word i occurs in document j.
We have clearly lost some information, since this representation does not take into
account the order of the words. However matrices are much easier to work with, and
the underlying assumption is that it should still be possible to cluster the documents
just knowing what words each one contains but not their order. This is often called
the bag-of-words assumption.
The idea behind latent semantic indexing is to compute the singular value
decomposition of M and use this for information retrieval and clustering. More
precisely, if we write
M ≈ U (k) Σ(k) V (k)T
where U (k) is the first k columns of U , etc. then the columns of U (k) are the k
directions that maximize the projected variance of a random document. These
8
CHAPTER 2. NONNEGATIVE MATRIX FACTORIZATION
vectors are interpreted as “topics”. More precisely, suppose we want to compute a
“similarity” score for document i and document j. We could do this by computing
hMi , Mj i
where Mi is the ith column of M , etc. This function “counts” the number of words
in common. In particular, given a query we would judge how similar a document is
to it just be counting how many of its words occur in each document. This is quite
naive. Instead, we could compute
hMiT U (k) , MjT U (k) i
Intuitively this maps each document to a vector of length k that measures how
much of each topic is present in the document, and computes the similarly of the
documents by taking an inner-product in this low-dimensional space. In practice
this is a much better way to measure similarity and was introduced by the seminal
paper of Deerwester et al [49].
However it has its own weaknesses. This approach has some rather undesirable
properties:
(a) “topics” are orthonormal
Consider topics like “politics” and “finance”. Are the sets of words that describe
these topics uncorrelated? No!
(b) “topics” contain negative values
This is more subtle, but negative words can be useful to signal that document is
not about a given topic. But then when we compute similarly, two documents are
judged to be more similar based on a topic that they are both decidedly not about.
This is another counter intuitive and undesirable property.
Nonnegative Matrix Factorization
The idea due to [73] and [98] is to write
M ≈ AW
where A and W are m × k and k × n respectively and are required to be entry-wise
nonnegative. In fact, let us suppose that the columns of M each sum to one. It is
not hard to see that if D is a diagonal matrix where the ith entry is the reciprocal
2.1. INTRODUCTION
9
eW
f where A
e = AD and
of the sum of the entries in the ith column of A then M = A
−1
f = D W normalizes the data so that the columns of A
e and of W
f each sum to one.
W
e
Hence we are finding a set of topics (the columns of A which are each distributions
on words) so that every document can be obtained as a convex combination of the
topics that we have found.
This optimization problem plays a crucial role in many machine learning systems, such as image segmentation, text analysis, recommendation systems, etc. But
this optimization problem is N P -hard [115]. So what should we do now? Give up?
In contrast, singular value decomposition is a problem where theory and practice agree! It can be computed efficiently, and it has many uses. But in spite of this
intractability result, nonnegative matrix factorization really is used in practice. The
standard approach is to use alternating minimization:
Alternating Minimization: This problem is non-convex, but suppose we
guess A. Then computing the nonnegative W that minimizes kM −AW kF is convex
and can be solved efficiently. The approach is to guess A, compute the best W then
set W as fixed and compute the best A, and so on. This process converges, but not
necessarily to the optimal solution.
It can and does get stuck in local minima in practice!
We note that this approach is also called expectation-maximization [50], and is the
standard approach not just for nonnegative matrix factorization, but for many other
problems we will study in this course such as dictionary learning and learning mixtures models.
Food for Thought
But maybe heuristics like this are identifying interesting instances of the problem.
The goal of this course is to not give up when faced with intractability, and to
look for new explanations. These explanations could be new models (that avoid the
aspects of the problem that allow us to embed hard problems) or could be identifying
conditions under which heuristics that are already used, do work. This is a largely
unexplored area.
In the next section, we will ask what happens if we restrict the number of
topics. The instances generated by [115] have k linear in m and n, but when we
look for a set of topics that explain 300, 000 New York Times articles, we are looking
for only a few hundred topics. So one way to reformulate the question is to ask
what its complexity is as a function of k. We will essentially resolve this using
algebraic techniques. Nevertheless if we want even better algorithms, we need more
10
CHAPTER 2. NONNEGATIVE MATRIX FACTORIZATION
assumptions. We will see how a geometric interpretation of this problem implies that
these hard instances are unstable, and we will examine a condition (separability)
that enforces stability, and allows us to give much better algorithms - ones that run
in time polynomial in all of the parameters.
2.2
Algebraic Algorithms
In the previous section we introduced the nonnegative matrix factorization problem
and described its applications to text analysis (it has many other applications).
Vavasis proved that this problem is N P -hard in the worst-case, but the instances
he contracted have k – the number of topics – linear in the size of the matrix [115].
In most practical applications, k is much smaller than m or n and with this in mind
we will instead ask: What is the complexity of this problem as a function of k?
We will make use of tools from algebra to give a polynomial time algorithm for any
k = O(1). In fact, the algorithm we present here will be nearly optimal in terms of
its dependence on k.
Definitions
Let us define the nonnegative matrix factorization problem formally, since we did
so only informally in the previous section: Suppose we are given an entry-wise
nonnegative matrix M of size m × n.
Definition 2.2.1 The nonnegative rank of M – denoted by rank+ (M )– is the smallest k such that there are nonnegative matrices A and W of size m × k and k × n
respectively that satisfy M = AW .
Equivalently, rank+ (M ) is the smallest
P k such that there are k nonnegative rank
one matrices {Mi } that satisfy M = i Mi .
Both of these equivalent formulations of the problem will be useful throughout
our discussion. To gain some familiarity with this parameter, it is helpful to compare
it to a more familiar one: If we omit the requirement that A and W be entry-wise
nonnegative, then the smallest k is precisely the rank of M . Hence the following
relation is immediate:
Fact 2.2.2 rank+ (M ) ≥ rank(M )
In fact the rank and the nonnegative rank of a matrix can be quite different:
2.2. ALGEBRAIC ALGORITHMS
11
Example. Let M ∈ Mn×n , where Mij = (i − j)2 . It is easy to see that the
columns of M are spanned by
     

1
12 
 1


1  2   22 

     
 ..  ,  ..  ,  .. 

 .   .   . 




 1
n
n2 
It is easy to see that rank(M ) = 3 However, M has zeros along the diagonal and
non-zeros off it. Furthermore for any rank one nonnegative matrix Mi , its pattern
of zeros and non-zeros is a combinatorial rectangle - i.e. the intersection of some set
of rows and columns - and a standard argument implies that rank+ (M ) = Ω(log n).
There are examples with even larger separations too.
Next we will connect nonnegative matrix factorization to computational problems involving systems of polynomial inequalities.
Systems of Polynomial Inequalities
We can reformulate the problem of finding an A and W that prove rank+ (M ) ≤ k
as a problem of finding a feasible solution to a particular system of polynomial
inequalities. More specifically, the problem we want to solve is:


M = AW
(2.1)
A ≥0


W ≥0
This system consists of quadratic equality constraints (one for each entry of M ),
and linear constraints that A and W be entry-wise nonnegative. Before trying to
design better algorithms for k = O(1), we should ask a more basic question (whose
answer is not at all obvious):
Question 3 Is there any finite time algorithm?
The difficulty is that even if there is a solution, the entries of A and W could be
irrational. This is quite different than, say, 3-SAT where there is a simple brute-force
algorithm. In contrast for nonnegative matrix factorization it is quite challenging
to design algorithms that run in any finite amount of time. But indeed there are
algorithms (that run in some fixed amount of time) to decide whether a system
of polynomial inequalities has a solution or not in the real RAM model. These
algorithms can also compute an implicit representation of the solution, if there is
12
CHAPTER 2. NONNEGATIVE MATRIX FACTORIZATION
one. The output is a polynomial and an interval (for each variable) in which there
is only one root, which is the value of the variable in the true solution. And you can
find as many bits of the solution as you would like by performing binary search for
the root.
The first algorithm follows from the seminal work of Tarski, and there has
been a long line of improvements based on successively more powerful algebraic
decompositions. This line of work culminated in algorithms whose running time is
exponential in the number of variables but is polynomial in all the other parameters
of the problem (the number of polynomial inequalities, the maximum degree and
the bit complexity of the coefficients). The running time is (nD)O(r) where n is the
number of polynomial inequalities, D is the maximum degree and r is the number of
variables [106]. This running time is essentially optimal under the exponential time
hypothesis [78]. In particular, if there is an algorithm for this problem that runs in
time (pD)o(r) then it would yield sub-exponential time algorithms for 3-SAT.
We can use these algorithms to solve nonnegative matrix factorization. However the number of variables we would need in the naive representation is nk + mk,
one for each entry in A or W . So even if k = O(1), we would need a linear number of
variables and the running time would be exponential. However we could hope that
even though the naive representation uses many variables, perhaps there is a more
clever representation that uses many fewer variables. Can we reduce the number of
variables in the system of polynomial inequalities from O(nk + mk) to f (k)?
If we could do this, then we could solve nonnegative matrix factorization in
polynomial time for any k = O(1). Next, we will describe some basic tools in the
first-order theory of the reals. These results will help formalize our intuition from
above that the number of variables is the right complexity measure when reasoning
about how difficult it is to solve a system of polynomial inequalities, but their proof
is out of scope of this course.
First-Order Theory of the Reals
Definition 2.2.3 A set S is semialgebraic if there exist multivariate polynomials
p1 , ..., pn such that
S = {x1 , ..., xr |pi (x1 , ..., xr ) ≥ 0}
or if S is a finite union or intersection of such sets.
Definition 2.2.4 The projection of a semialgebraic set S is defined as
projS (X1 , ..., X` ) = {x1 , ..., x` |∃ x`+1 , ..., xr such that p(x1 , ..., xr ) ∈ S}
2.2. ALGEBRAIC ALGORITHMS
13
Theorem 2.2.5 (Tarski) The projection of a semialgebraic set is semialgebraic.
This is one of the foundational results in the field, and is often called quantifier
elimination [110], [107]. To gain some familiarity with this notion, consider the
case of algebraic sets (defined analogously as above, but with polynomial equality
constraints instead of inequalities). Indeed, the above theorem implies that the
projection of an algebraic set is itself semi-algebraic. Is its projection also algebraic?
No (e.g. think about the projection of a circle)!
Earlier, we stated that there are algorithms to solve systems of polynomial
inequalities (and find an implicit representation for the solution, if there is one) in
time (nD)O(r) where n is the number of polynomial inequalities, D is the maximum
degree and r is the number of variables [106]. In fact, these algorithms work in a more
general setting where there is additionally a boolean function B that constraints the
sign pattern of the polynomials. We are interested in deciding whether the set
S = {x1 , ..., xr |B(p1 (x1 , ..., xr ), ..., pn (x1 , ..., xr )) = true}
is non-empty, and we assume that we can evaluate B (but not, say, that it has a
succinct circuit). A related result is the famous Milnor-Warren bound (see e.g. [7]):
Theorem 2.2.6 (Milnor-Warren) Given n polynomials p1 , ..., pm of degree ≤ D
on r variables x = x1 , ...xr , consider the sign pattern at x:
x → sgn(p1 (x)), sgn(p2 (x)), ..., sgn(pm (x))
Then as x ranges over Rr the number of distinct sign patterns is at most (nD)r .
A priori we could have expected as many as 3n sign patterns. In fact, algorithms
for solving systems of polynomial inequalities are based on cleverly enumerating the
set of sign patterns so that the total running time is dominated by the maximum
number of distinct sign patterns that there could be! In fact, the Milnor-Warren
bound can be thought of as an analogue of the Sauer-Shelah lemma that is used
throughout supervised learning where the number of variables plays the role of the
V C-dimension.
Next we will give a technique to reduce the number of variables.
Variable Reduction
It is clear that the set of points satisfying (2.1) is a semialgebraic set. However even
for k = 3 this system has a linear (in n and m) number of variables, so directly
solving (2.1) would require exponential time.
14
CHAPTER 2. NONNEGATIVE MATRIX FACTORIZATION
Question 4 Can we find an alternate system of polynomial inequalities that expresses the same decision problem but uses many fewer variables?
We will focus on a special case called simplicial factorization where rank(M ) = k.
In this case, we are asking whether or not rank+ (M ) = rank(M ) = k and this
simplifies matters because of the following observation:
Claim 2.2.7 In any solution, A and W must have full column and row rank respectively.
Proof: The span of the columns of A must contain the columns of M and similarly
the span of the rows of W must contain the rows of M . Since rank(M ) = k and
A and W have k columns and rows respectively we conclude that the A and W
must have full column and row rank respectively. Moreover their span must be the
column space and row space of M respectively. Hence we know that A and W have left and right pseudo-inverses A+ and W +
respectively. We will make use of these pseudo-inverses to reduce the number of
variables in our system of polynomial inequalities: We have that A+ A = Ir where
Ik is the k × k identity. Hence
A+ AW = W
and so we can recover the columns of W from a linear transformation of the columns
of M . This leads to the following alternative system of polynomial inequalities:
(2.2)

+ +

M W A M
MW +

 +
A M
=M
≥0
≥0
A priori, it is not clear that we have made progress since this system also has
nk + mk variables corresponding to the entries of A+ and W + . However consider
the matrix A+ M . If we represent A+ as an k × n matrix then we are describing
its action on all vectors, but the crucial observation is that we only need to know
how A+ acts on the columns of M which span a k dimensional space. Hence we can
apply a change of basis to rewrite M as MR which is an k × m matrix, and there
is an k × k linear transformation T (obtained from A+ and the change of basis) so
that T MR = W . A similar approach works for W , and hence we get a new system:


MC ST MR = M
(2.3)
MC S
≥0


T MR
≥0
2.3. STABILITY AND SEPARABILITY
15
The variables of this system are the entries in S and T . So there are 2k 2
variables. And the properties we need of this system are that
(a) If the simplicial factorization problem has a solution, then there is a solution
to this system (completeness)
(b) If there is any solution to the system, then the simplicial factorization has a
solution (soundness)
We have already proven the first property, and the second property follows
because we can set A = MC S and W = T MR and this is a valid factorization with
inner-dimension k. Hence if we apply Renegar’s algorithm to this new system, the
2
algorithm runs in time (nm)O(k ) and solves the simplicial factorization problem.
The above approach is based on the paper of Arora et al [13] where the authors
also give a variable reduction procedure for nonnegative matrix factorization (in the
general case where A and W need not have full column or row rank respectively).
The authors reduce the number of variables from (nk + mk) to f (k) = 2k 2 2k and
this yields a doubly-exponential time algorithm as a function of k. The crucial
observation is that even if A does not have full column rank, we could write a system
of polynomial inequalities that has a pseudo-inverse for each set of its columns
that
k
is full rank (and similarly for W ). However A could have as many as k/2
maximal
sets of linearly independent columns, and hence the resulting system of polynomial
inequalities has f (k) variables but f (k) is itself exponential in k.
In [94] the author further reduces the number of variables to 2k 2 for nonnegative matrix factorization, and the main idea is that even though A could have
exponentially many maximal sets of linearly independent columns, their psueudoinverses are algebraically dependent and can be expressed over a common set of k 2
variables using Cramer’s rule. This yields a singly exponential time algorithm for
2
nonnegative matrix factorization that runs in (nm)O(k ) time which is essentially optimal since any algorithm that runs in time (nm)o(k) would yield a sub-exponential
time algorithm for 3-SAT [13].
2.3
Stability and Separability
In the previous section we took an algebraic approach and here instead we will work
with an assumption called separability [54] which will allow us to give an algorithm
that runs in polynomial time (even for large values of r). Our discussion will revolve
around the intermediate simplex problem.
16
CHAPTER 2. NONNEGATIVE MATRIX FACTORIZATION
Intermediate Simplex Problem
Let us define the intermediate simplex problem:
We are given two polytopes Q and P with P ⊆ Q and furthermore P is
encoded by its vertices and Q is encoded by its facets. Is there a simplex
K with P ⊆ K ⊆ Q?
We would like to connect this problem to nonnegative matrix factorization, since
it will help us build up a geometric view of the problem. Consider the following
problem:
Given nonnegative matrices M and A, does there exists W ≥ 0 such
that M = AW ?
The answer is “Yes”, if and only if each column of M is in the cone spanned
by nonnegative combinations of the columns of A. Moreover if we normalize the
columns of M and A so that they sum to one, then the answer is “Yes” if and only if
the convex hull of the columns of A contains the columns of M . Recall in simplicial
factorization we are given a nonnegative matrix M with rank(M ) = k, and our
goal is to decide whether or not rank+ (M ) = k. We will prove that the simplicial
factorization problem and the intermediate simplex problem are equivalent [115].
Consider the following helper problem, which we call (P0):
Given M = U V , is there an invertible k × k matrix T such that U T −1 ,
and T V are nonnegative?
In fact, Vavasis [115] proved that (P0), intermediate simplex and the simplicial
factorization problem are each polynomial time interreducible. It is easy to see
that (P0) and the simplicial factorization problem are equivalent since in any two
factorizations M = U V or M = AW (where the inner-dimension equals the rank of
M ), the column spaces of M , U and A are identical. Similarly the rows spaces of
M , V and W are also identical.
The more interesting aspect of the proof is the equivalence between (P0) and
the intermediate simplex problem. The translation is:
(a) rows of U ⇐⇒ vertices of P
(b) rows of T ⇐⇒ vertices of K
(c) columns of V ⇐⇒ facets of Q
2.3. STABILITY AND SEPARABILITY
17
Figure 2.1: This figure is taken from [115]. The intermediate simplex problem has
two solutions which will be used to encode the truth assignment of a variable.
Then the constraint that U T −1 is nonnegative is (roughly) the constraint that P ⊆ K
and the constraint T V is (roughly) the constraint K ⊆ Q. There are some tedious
normalization issues that arise since we need to be careful about the distinction
between the convex hull of a set of vectors and the cone generated by all nonnegative
combinations. However this equivalence gives us a geometric view that will be
helpful.
Vavasis made use of the equivalences in the previous subsection to prove that
nonnegative matrix factorization is N P -hard. Consider the gadget in Figure 2.1;
the crucial property is that there are only two possible intermediate triangles, which
can then be used to represent the truth assignment for a variable xi . The description
of the complete reduction, and the proof of its soundness are involved (see [115]).
The trouble is that gadgets like those in Figure ?? are unstable. We can change
the number of solutions by small perturbations to the problem. Motivated by issues of
uniqueness and robustness, Donoho and Stodden [54] introduced a condition called
separability that alleviates many of these problems, which we will discuss in the next
subsection.
Separability
Definition 2.3.1 We call A separable if, for every column of A, there exists a row
of A whose only non-zero entry is in that column.
18
CHAPTER 2. NONNEGATIVE MATRIX FACTORIZATION
Furthermore in the separable nonnegative matrix factorization problem we are
given M and the promise that if there is a nonnegative matrix factorization, there is
one in which A is separable. Donoho and Stodden used this condition (and others)
to show that there are somewhat natural conditions under which the nonnegative
matrix factorization is unique. Arora, Ge, Kannan and Moitra gave an algorithm
for finding it:
Theorem 2.3.2 [13] Given a nonnegative matrix M with the promise that there is
a nonnegative matrix factorization M = AW where A is separable, there is a polynomial time algorithm to compute such a factorization of minimum inner-dimension.
In fact, separability is quite natural in the context of text analysis. Recall that
we interpret the columns of A as topics. We can think of separability as the promise
that these topics come with anchor words; informally, for each topic there is an
unknown anchor word that if it occurs in a document, the document is (partially)
about the given topic. For example, 401k could be an anchor word for the topic
personal finance.
Why do anchor words help? It is easy to see that if A is separable, then the
rows of W appear as rows of M (after scaling). Hence we just need to determine
which rows of M correspond to anchor words. We know from our discussion in
Section 2.3 that (if we scale M , A and W so that their rows sum to one) the convex
hull of the rows of W contain the rows of M . But since these rows appear in M as
well, we can try to find W by iteratively deleting rows of M that do not change its
convex hull.
Let M i denote the ith row of M and let M I denote the restriction of M to
the rows in I for I ⊆ [n]. So now we can find the anchor words using the following
simple procedure:
Find Anchors [13]
Input: matrix M ∈ Rn×m satisfying the conditions in Theorem 2.3.2
Output: W = M I
Set I = [n]
For i = 1, 2, ..., n
If M i ∈ conv({M j |j ∈ I, j 6= i}), set I ← I − {i}
End
It is easy to see that deleting a row of M that is not an anchor word will not
change the convex hull of the remaining rows, and so the above algorithm terminates
2.4. TOPIC MODELS
19
with a set I that only contains anchor words. Moreover at termination
conv({M i |i ∈ I}) = conv({M j }j )
Alternatively the convex hull is the same as at the start. Hence the anchor words
that are deleted are redundant and we could just as well do without them.
Separable NMF [13]
Input: matrix M ∈ Rn×m satisfying the conditions in Theorem 2.3.2
Output: A, W
Run Find Anchors on M , let W be the output
Solve for nonnegative A that minimizes kM − AW kF (convex programming)
End
The proof of theorem follows immediately from the proof of correctness of
Find Anchors and the fact that conv({M i }i ) ⊆ conv({W i }i ) if and only if there
is a nonnegative A (whose rows sum to one) with M = AW .
The above algorithm when naively implemented would be prohibitively slow.
Instead, there have been many improvements to the above algorithm [27], [84] [65],
and we will describe one in particular that appears in [12]. Suppose we choose a
row M i at random. Then it is easy to see that the furthest row from M i will be an
anchor word.
Similarly, if we have found one anchor word the furthest row from it will be
another anchor word, and so on. In this way we can greedily find all of the anchor
rows, and moreover this method only relies on pair-wise distances and projection
so we can apply dimension reduction before running this greedy algorithm. This
avoids linear programming altogether in the first step in the above algorithm, and
the second step can also be implemented quickly because it involves projecting a
point into an k − 1-dimensional simplex.
2.4
Topic Models
Here we will consider a related problem called topic modeling; see [28] for a comprehensive introduction. This problem is intimately related to nonnegative matrix factorization, with two crucial differences. Again there is some factorization M = AW
f which is a very crude approxibut now we do not get access to M but rather M
mation. Intuitively, each column in M is a document that is itself a distribution on
20
CHAPTER 2. NONNEGATIVE MATRIX FACTORIZATION
words. But now the words that we observe are samples from this distribution (so
we do not actually know the columns of M ).
The second difference is that we think of W as stochastically generated. There
are in fact many popular choices for this distribution:
(a) Pure Documents: Each document is about only one topic, hence each column of W has exactly one non-zero.
(b) Latent Dirichlet Allocation [30] : The columns of W are generated from
a Dirichlet distribution.
(c) Correlated Topic Model [29] : Certain pairs of topics are allowed to be
positively or negatively correlated, and the precise distribution that generates
the columns of W is log-normal.
(d) Pachinko Allocation Model [89] : This is a multi-level generalization of
LDA that also allows for certain types of structured correlations.
f. To
There are many more choices. Regardless, our goal is to learn A from M
emphasize the differences, note that even if we knew A we cannot compute W
f and M can be quite different since the former may be
exactly. Alternatively, M
sparse while the latter is dense. Are there provable algorithms for topic modeling?
The Gram Matrix
We will follow an approach of Arora, Ge and Moitra [14]. At first this seems like a
fundamentally different problem than the ones we have considered because in this
model we cannot ask for longer documents, we can only ask for more of them. Hence
f but each column is not that close to
we are increasing the number of columns of M
the corresponding column in M . The basic idea is to work instead with the Gram
matrix G:
Definition 2.4.1 Let G denote the word × word matrix whose entry in (a, b) is
the probability that the first two words in a randomly chosen document are a and b
respectively.
Definition 2.4.2 Let R denote the topic × topic matrix whose entry in (i, j) is
the probability that the first two words (again in a randomly chosen document) are
generated from the topics i and j respectively.
2.4. TOPIC MODELS
21
Note that we can approximate G from our samples, however we cannot (directly) approximate R and it is controlled by the choice of which distribution we
use to generate the columns of W . More precisely:
Lemma 2.4.3 G = ARAT
Proof: Let w1 denote the first word and let t1 denote the topic of w1 (and similarly
for w2 ). We can expand P[w1 = a, w2 = b] as:
X
P[w1 = a, w2 = b|t1 = i, t2 = j]P[t1 = i, t2 = j]
i,j
and the lemma is now immediate. The key observation is that G has a separable nonnegative matrix factorization
given by A and RAT since A is separable and the latter matrix is nonnegative. Indeed
if RAT has full row rank then the algorithm in Theorem 2.3.2 will find the true set
of anchor words. However since the rows of RAT are no longer normalized to sum to
one, the above factorization is not necessarily unique. Nevertheless we have made
some progress, and we can adopt a Bayesian interpretation (see [12]).
Recovery via Bayes Rule
In fact, the entries of A are conditional probabilities P(w1 |t1 ) and so we can reason
about the posterior distribution P(t1 |w1 ). In fact this gives us an alternate characterization of an anchor word: A word is an anchor word if and only if its posterior
distribution is supported on just one topic. In particular
(
1,
w is an anchor word for t,
P(t1 = t|w1 = w) =
0,
otherwise,
Now we can expand:
P (w1 = w0 |w2 = w) =
X
P(w1 = w0 |w2 = w, t2 = t) · P(t2 = t|w2 = w),
t
In fact w1 is independent of w2 if we condition on t2 and so:
P(w1 = w0 |w2 = w, t2 = t) = P(word1 = w0 |topic2 = t)
= P(word1 = w0 |word2 = anchor(t)),
22
CHAPTER 2. NONNEGATIVE MATRIX FACTORIZATION
which we can compute from G after having determined the anchor words. Hence:
X
P(w1 = w0 |w2 = w) =
P(word1 = w0 |word2 = anchor(t))P(t2 = t|w2 = w)
t
which we can think of a linear systems in the variables {P(t2 = t|w2 = w)}. It is
not hard to see that if R has full rank then it has a unique solution. Finally, we
compute the probabilities we were originally interested in by Bayes’ rule:
P(topic t|word w) · P(word w)
P(topic t)
P(topic t|word w) · P(word w)
.
=P
0
0
w0 P(topic t|word w ) · P(word w )
P(word w|topic t) =
e be the empirical Gram matrix,
We can now state the algorithm Recover. Let G
e
where Ga,b is the fraction of documents in our sample whose first word is a and
whose second word is b.
Suppose each anchor word has probability at least p. Then the main result in
this subsection is:
Theorem 2.4.4 [14] For any separable topic model where R is full rank there is
e that is ε-close to A and the running
a polynomial time algorithm to compute A
time and sample complexity (number of documents) is poly(n, 1/p, 1/ε, 1/σmin (R)),
provided documents have length at least two.
In the next subsection we describe some experimental results.
Recover [14], [12]
Input: term-by-document matrix M ∈ Rn×m
Output: A, R
e compute P(w1 = w|w2 = w0 )
Compute G,
Run Find Anchors
Solve for P(topic t|word w) and use Bayes’ rule to compute A
End
Experiments
We are faced with a basic scientific question now: Are there really anchor words?
The following experiment was conducted in [12]:
2.4. TOPIC MODELS
23
(a) Run MALLET (a popular topic modeling toolkit) on a collection of New York
Times articles, its output is a topic matrix A.
(b) Use A to generate data from a topic model, run MALLET on this data.
The important point is that here the data that we are running on is actually from a
topic model and we can compare how well one algorithm can recover the true matrix
compared to how well another algorithm does. Then:
(c) Run the new algorithm on this data.
This is a seemingly unfair comparison, since we have restricted ourselves to a
topic matrix A that MALLET has already found once (so this is our notion of what
constitutes a realistic topic model). Yet surprisingly the algorithm in the previous
subsection was able to find the topic matrix A more accurately and orders of magnitude faster! This is an important example where finding conditions under which
we can give provable algorithms indeed led to much better algorithms in practice.
24
CHAPTER 2. NONNEGATIVE MATRIX FACTORIZATION
Chapter 3
Tensor Methods
In this chapter we will study algorithms for tensor decompositions and their applications to statistical inference.
3.1
Basics
Here we will introduce the basics of tensors. A matrix is an order two tensor – it is
indexed by a pair of numbers. In general a tensor is indexed over k-tuples, and k is
called the order of a tensor. We can think of a tensor T as a point in Rn1 ×n2 ×...×nk .
We will mostly be interested in order three tensors throughout this chapter. If T is
an order three tensor of size m × n × p we can regard T as a collection of p matrices
of size m × n that are stacked on top of each other.
We can generalize many of the standard definitions from linear algebra to the
tensor setting, however we caution the reader that while these parameters are easy
to compute for matrices, most parameters of a tensor are hard to compute (in the
worst-case).
Definition 3.1.1 A rank one tensor is a tensor of the form T = u ⊗ v ⊗ w where
Ti,j,k = ui vj wk . And in general the rank of a tensor T is the minimum r such that
we can write T as the sum of r rank one tensors.
Question 5 Tensors are computationally more difficult to work with; so why should
we try to work with them?
In fact, we will give a motivating example in the next section that illustrates the
usefulness of tensor methods in statistics and machine learning (and where matrices
are not sufficient).
25
26
CHAPTER 3. TENSOR METHODS
Case Study: Spearman’s Hypothesis
Charles Spearman was a famous psychologist who postulated that there are essentially two types of intelligence: mathematical and verbal. In particular, he believed
that how well a student performs at a variety of tests depends only on their intrinsic
aptitudes along these two axes. To test his theory, he set up a study where a thousand students each took ten various types of test. He collected these results into a
matrix M where the entry Mi,j was used to denote how well student i performed on
test j. Spearman took the best rank two approximation to M . In other words, that
there exists vectors (not necessarily unit vectors) u1 , u2 ∈ R1000 , v1 , v2 ∈ R10 , such
that
M ≈ u1 v1T + u2 v2T
This is called factor analysis, and his results somewhat confirmed his hypothesis.
But there is a fundamental obstacle to this type of approach that is often referred
to as the “Rotation Problem”. Set U = [u1 , u2 ] and V = [v1 , v2 ] and let O be an
orthogonal matrix. Then
U V T = U O OT V T
is an alternative factorization that approximates M just as well. However the
columns of U O and the rows of OT V T could be much less interpretable. To summarize, just because there is a good factorization of a given data matrix M does not
mean that factor analysis will find it.
P
Alternatively, suppose we are given a matrix M = ri=1 xi yiT .
Question 6 Can we determine {xi }i and {yi }i if we know M ?
Actually, there are only trivial conditions under which we can uniquely determine
these factors. If r = 1 of if we know for a priori reasons that the vectors {xi }i and
{yi }i are orthogonal, then we can. But in general we could take the singular value
decomposition of M = U ΣV T and take {σi ui }i and {vi }i to be an alternative set
of factors that explain M (and if {xi }i and {yi }i are not orthogonal, then these are
clearly two different sets of factors for the same M ).
However if we are given a tensor
T =
r
X
xi ⊗ y i ⊗ w i
i=1
then there are general conditions (namely if {xi }i , {yi }i and {wi }i are each linearly
independent) not only is the true factorization the unique factorization of T with
rank r but in fact there are simple algorithms to find it! This is precisely the reason
that tensor methods are ubiquitous in statistics and machine learning: If we are
3.1. BASICS
27
given a tensor whose factors represent the parameters of a statistical model, we can
find these factors efficiently; yet for matrices the factors are not uniquely determined.
Complexity of Tensor Problems
In the previous subsection, we alluded to the fact that tensor methods will offer a
way around the “Rotation Problem” which is a common obstacle in factor analysis.
So can we just compute the minimum rank decomposition of a tensor? In fact, not
only is this problem computationally hard (without further assumptions) but most
tensor problems are hard [71]! Even worse, many of the standard relations in linear
algebra do not hold and even the definitions are in some cases not well-defined.
(a) For a matrix A, dim(span({Ai }i )) = dim(span({Aj }j )) (the column rank
equals the row rank).
However no such relation holds for tensors.
(b) For a matrix A, the best rank k approximation to A can be obtained from its
best rank k + 1 approximation.
In particular, if we let A(k+1) be the best rank k + 1 approximation to A, then the
best rank k approximation to A(k+1) is the best rank k approximation to A. But for
tensors the best rank k and rank k + 1 approximations do not necessarily share any
common rank one factors. In fact, subtracting the best rank one approximation to
a tensor T from it can actually increase its rank.
(c) For a real-valued matrix its rank over R and over C are the same, but this is
false for tensors.
There are real-valued tensors whose minimum rank decomposition requires complex
numbers.
Perhaps the most worrisome issue is that in some cases the definitions fail too:
Definition 3.1.2 The border rank of a tensor T is the minimum r such that for
any ε > 0 there is a rank r tensor that is entry-wise ε close to T .
We remark that what norm we use in the above definition is not important. In
fact, for matrices the border rank is equal to the rank. But for tensors these can be
different.
28
CHAPTER 3. TENSOR METHODS
(d) For a tensor, its border rank is not necessarily equal to its rank.
Consider the following 2 × 2 × 2 tensor T , over R:
0 1
1 0
T =
,
.
1 0
0 0
We will omit the proof that T has rank 3, but show that T admits an arbitrarily
close rank 2 approximation. Consider the following matrices
n 0
0 0
1 n1
n 1
and Rn =
,
.
, 1 1
Sn =
0 0
0 0
1 n1
n
n2
1
1
1
, and hence is rank 1, and
⊗ 1/n
⊗ 1/n
It is not too hard to see that Sn = n 1/n
Rn is also rank 1. Thus the tensor Sn − Rn is rank 2, but also is an 1/n entry-wise
approximation of T .
One last issue is that it is easy to see that a random n × n × n tensor will have
rank Ω(n2 ), but it is unknown how to explicitly construct any order three tensor
whose rank is Ω(n1+ε ). And any such construction would give the first super-linear
circuit lower bounds for any explicit problem [102] which is a long-standing open
question in circuit complexity.
Jennrich’s Algorithm
While we cannot hope for algorithms that find the minimum rank decomposition of
a tensor in general, in fact there are mild conditions under which we can do it. This
algorithm has been rediscovered numerous times in a wide range of applications, and
after an extensive search we discovered that this simple algorithm was first reported
in a working paper of Harshman [70] where the author credits Dr. Robert Jennrich.
We will state and prove a version of this result that is more general, following the
approach of Leurgans, Ross and Abel [87]:
Theorem 3.1.3 [70], [87] Consider a tensor
T =
r
X
ui ⊗ vi ⊗ wi
i=1
where each set of vectors {ui }i and {vi }i are linearly independent, and moreover each
pair of vectors in {wi }i are linearly independent too. Then the above decomposition
is unique up to rescaling, and there is an efficient algorithm to find it.
3.1. BASICS
29
We will see a wide variety of applications of this basic result (which may explain why it has been rediscovered so many times) to phylogenetic reconstruction
[96], topic modeling [8] and community detection [9]. This decomposition also plays
a crucial role in learning mixtures of spherical Gaussians [75] and independent component analysis [36], although we will instead present a local search algorithm for
the latter problem.
Tensor Decomposition [70], [87]
Input: tensor T ∈ Rm×n×p satisfying the conditions in Theorem 3.1.3
Output: factors {ui }i , {vi }i and {wi }i
Choose a, b ∈ Sp−1 uniformly at random; set Ta = T (∗, ∗, a) and Tb = T (∗, ∗, b)
Compute the eigendecomposition of Ta (Tb )+ and Tb (Ta )+
Let U and V be the eigenvectors
Pair up ui and vi iff their eigenvalues are reciprocals
Solve for wi in T =
End
Pr
i=1
ui ⊗ vi ⊗ wi
Recall that Ta is just the weighted sum of matrix slices through T , each weighted
by ai . It is easy to see that:
Claim 3.1.4 Ta =
Pr
T
i=1 hwi , aiui vi
and Tb =
Pr
T
i=1 hwi , biui vi
Alternatively, let Da = diag({hwi , ai}i ) and let Db = diag({hwi , bi}i ). Then we can
write Ta = U Da V T and Tb = U Db V T where the columns of U and V are ui and vi
respectively.
Lemma 3.1.5 The eigenvectors of Ta (Tb )+ and Tb (Ta )+ are U and V respectively
(after rescaling)
Proof: We can use the above formula for Ta and Tb and compute
Ta (Tb )+ = U Da Db+ U +
Then almost surely over the choice of a and b we have that the diagonals of Da Db+
will be distinct – this is where we use the condition that each pair of vectors in {wi }i
is linearly independent.
30
CHAPTER 3. TENSOR METHODS
Hence the above formula for Ta (Tb )+ is an eigendecomposition, and moreover
it is unique because its eigenvalues are distinct. We conclude that the eigenvectors
of Ta (Tb )+ are indeed the columns of U up to rescaling, and similarly for V . Now to complete the proof of the theorem, notice that ui and vi as eigenvectors of Ta (Tb )+ and Tb (Ta )+ respectively, have eigenvalues of (Da )i,i (Db )−1
i,i and
−1
+
(Db )i,i (Da )i,i . (Again, the diagonals of Da (Db ) are distinct almost surely and so
vi is the only eigenvector that ui could be paired with). Since we only have the
factors ui × vi up to scaling, we will need to push the rescaling factor in with wi .
Nevertheless we just need to prove that linear system over the wi ’s does not have
more than one solution (it certainly has one).
Definition 3.1.6 The Khatri-Rao product ⊗KR between two matrices U and V
with the same number of columns is
U ⊗KR V = ui ⊗ vi
i
That is the Khatri-Rao product of U and V of size m × r and n × r is an mn × r
matrix whose ith column is the tensor product of the ith column of U and the ith
column of V . The following lemma we leave as an exercise to the reader:
Lemma 3.1.7 If U and V are size m × r and n × r and have full column rank and
r ≤ m + n − 1 then U ⊗KR V has full column rank too.
This immediately implies that the linear system over the wi ’s has a unique solution.
This completes the proof of the theorem.
Note that if T is size m×n×p then the conditions of the theorem can only hold
if r ≤ min(m, n). There are extensions of the above algorithm that work for higher
order tensors even if r is larger than any of the dimensions of its factors [48], [66],
[26] and there are interesting applications to overcomplete independent component
analysis [66] and learning mixtures of many Gaussians [26], [11].
In the next section, we will show that the above algorithm is stable – in all of
the applications in learning we will estimate T from our samples and hence we do
not have T exactly.
3.2
Perturbation Bounds
In the last section, we gave an algorithm for tensor decomposition when the factors are full-rank, and in this setting its decomposition is unique (up to rescaling).
3.2. PERTURBATION BOUNDS
31
However in all of our applications we will not be given T exactly but rather we will
compute an approximation to it from our samples. Our main goal in this section is
to show that even in the presence of noise, the algorithm in Theorem 3.1.3 recovers
factors close to the true factors. In later sections, we will simply assume we are given
the true tensor T and what we present here is what justifies this simplification.
This section is somewhat technical, and the reader can feel free to skip it.
Recall that the main step in Theorem 3.1.3 is to compute an eigendecomposition. Hence our first goal is to establish conditions under which the eigendecomposition itself is stable. More precisely, let M = U DU −1 , where D is a diagonal
f = M + E, when can we recover good estimates to U ?
matrix. If we are given M
Intuitively, if any of the diagonal entries in D are close or if U is ill-conditioned,
then even a small perturbation E can drastically change the eigendecomposition. We
will prove that these are the only things that can go wrong. There will be two main
f is diagonalizable, and then we can show that
steps. First we need to prove that M
the matrix that diagonalizes it must be close to the one that diagonalizes M .
Condition Number
Definition 3.2.1 The condition number of a matrix M is defined as
κ(M ) :=
σmax (M )
,
σmin (M )
where σmax (M ) and σmin (M ) are the maximum and minimum singular values of M ,
respectively.
Consider the basic problem of solving for x in M x = b. Suppose that we are
given M exactly, but we only know an estimate eb = b + e of b. Here e is an error
term. By solving the equation M x = b using eb instead of b, we obtain an estimate
x
e for x. How close is x
e to x?
We have x
e = M −1eb = x + M −1 e = x + M −1 (eb − b). So
kx − x
ek ≤
1
kb − ebk.
σmin (M )
Since M x = b, we also have kbk ≤ σmax (M )kxk. It follows that
kx − x
ek
σmax (M ) kb − ebk
kb − ebk
≤
= κ(M )
.
kxk
σmin (M ) kbk
kbk
In other words, the condition number controls the relative error when solving a linear
system.
32
CHAPTER 3. TENSOR METHODS
Gershgorin’s Disk Theorem
Recall our first intermediate goal is to show that M + E is diagonalizable, and we
will invoke the following theorem:
Theorem 3.2.2 The eigenvalues of a matrix M are all contained in the following
union of disks in the complex plane:
n
[
D(Mii , Ri )
i=1
where D(a, b) := {x | kx − ak ≤ b} ⊆ C and Ri =
P
j6=i
|Mij |.
Proof: Let (x, λ) be an eigenvector-eigenvalue pair (note that this is valid even
when M is not diagonalizable). Let i P
denote the coordinate
Pof x with the maximum
absolute value. Then M x = λx gives j Mij xj = λxi . So j6=i Mij xj = λxi −Mii xi .
We conclude:
X
xj X
|Mij | = Ri .
Mij ≤
|λ − Mii | = xi j6=i
j≤i
Thus λ ∈ D(Mii , Ri ). Part 1
Now let us return to the question posed at the beginning of the previous section: is
f diagonalizable? Consider
M
fU = D + U −1 EU.
U −1 M
f is diagonalizable proceeds as follows:
The proof that M
f and U −1 M
fU are similar matrices, they have the same set
Part (a) Since M
of eigenvalues.
fU = D + U −1 EU
Part (b) Moreover we can apply Theorem 3.2.2 to U −1 M
and if U is well-conditioned and E is sufficiently small, the radii will be much
smaller than the closest pair of diagonal entries in D. Hence we conclude that the
fU and also those of M
f are distinct, and hence the latter can
eigenvalues of U −1 M
be diagonalized.
Thanks to Santosh Vempala for pointing out an error in the original analysis;
see also [66] for a more detailed proof along these lines.
3.2. PERTURBATION BOUNDS
33
Part 2
f=U
eD
eU
e −1 . Now we can turn to our second intermediate goal, namely how
Let M
does this compare to the actual diagonalization M = U DU −1 ?
ei ) and (ui , λi ) are corresponding eigenvector-eigenvalue
More specifically, if (e
ui , λ
ei ) to (ui , λi )? Using the argument
f and M respectively, how close is (e
pairs for M
ui , λ
ei ≈ λi for each i. Furthermore, we assume that when
in Part 1 we know that λ
i 6= j, the eigenvalues of M have sufficient separation. It remains to check that
u
ei ≈ ui . Let
X
cj u j = u
ei .
j
f = M + E. Left-multiplying both sides of the equation above by M
f,
Recall that M
we get
X
X
ei u
ei )uj = −Ee
cj λj uj + Ee
ui = λ
ei . =⇒
cj (λj − λ
ui .
j
j
Let wjT be the jth row of U −1 . Left-multiplying both sides of the above equation by
wjT , we get
ei ) = −wT Ee
cj (λj − λ
ui .
j
Recall we have assumed that the eigenvalues of M are separated. Hence if E is
ei is bounded away from zero. Then we can
sufficiently small we have that λj − λ
bound the cj ’s and this implies that u
ei and ui are close.
We can qualitatively restate this as follows: Let δ be the separation between the closest pair of eigenvalues of M and let κ be the condition number of
e − Uk ≤
U . Then if kEk ≤ poly(1/n, 1/κ, δ) the norm of the error satisfies kU
poly(1/n, 1/κ, δ, kEk).
Back to Tensor Decompositions
We will introduce some notation to explain the application to tensor decompositions.
Let “→” signify that one matrix converges to another at an inverse polynomial rate
(as a function of the number of samples). For example, Te → T when Te represents
the empirical moments of a distribution (with bounded moments) and T represents
its true moments. Also Tea = Te(∗, ∗, a) → Ta and similarly for b.
We leave it as an exercise to the reader to check that Teb+ → Tb+ under natural
conditions. It follows that Tea Teb+ → Ta Tb+ . We have already established that if
E → 0, then the eigendecompositions of M and M + E converge. Finally we
e and Ve which
conclude that the algorithm in Theorem 3.1.3 computes factors U
34
CHAPTER 3. TENSOR METHODS
converge to the true factors U and V at an inverse polynomial rate, and a similar
f and W as well.
proof works for W
Open Problem: Kruskal rank
We conclude this section with an open problem.
Definition 3.2.3 The Kruskal rank of a set of vectors {ui }i is the maximum r such
that all subset of r vectors are linearly independent.
We will see later that it is N P -hard to compute the Kruskal rank. Nevertheless, there
are strong uniqueness theorems for tensor decompositions (based on this parameter)
for which there is no known algorithmic proof:
P
Theorem 3.2.4 (Kruskal) Let T = ri=1 ui ⊗ vi ⊗ wi and let ku , kv and kw be the
Kruskal ranks of {ui }i , {vi }i , and {wi }i respectively. If ku + kv + kw ≥ 2r + 2 then
T has rank r and this decomposition of T is unique up to rescaling.
Open Question 1 Is there an efficient algorithm for tensor decompositions under
any natural conditions, for r = (1 + ε)n for any ε > 0?
For example, it is natural to consider a smoothed analysis model for tensor decomposition [26] where the factors of T are perturbed and hence not adversarially chosen.
The above uniqueness theorem would apply up to r = 3/2n − O(1) but there are no
known algorithms for tensor decomposition in this model for r = (1 + )n (although
there are much better algorithms for higher-order tensors).
3.3
Phylogenetic Trees and HMMs
Here we describe an application of tensor decompositions to phylogenetic reconstruction and HMMs.
The Model
A phylogenetic model has the following components:
(a) A rooted binary tree with root r, where the leaves do not necessarily have the
same depth.
3.3. PHYLOGENETIC TREES AND HMMS
35
The biological interpretation is that the leaves represent extant species (ones that
are still living), and the internal nodes represent speciation events.
(b) A set Σ of states, for example Σ = {A, C, G, T }. Let k = |Σ|.
(c) A Markov model on the tree; i.e. a distribution πr on the state of the root and
a transition P uv matrix for each edge (u, v).
We can generate a sample from the model as follows: We choose a state for the
root according to πr and for each node v with parent u we choose the state of v
according to the distribution defined by the ith row of P uv , where i is the state of
u. Alternatively, we can think of s(·) : V → Σ as a random function that assigns
states to vertices where the marginal distribution on s(r) is πr and
Pijuv = P(s(v) = j|s(u) = i),
Note that s(v) is independent of s(t) conditioned on s(u) whenever the (unique)
shortest path from v to t in the tree passes through u.
Our goal is to learn the above model - both the tree and the transition matrices - from a polynomial number of random samples. We will assume that the
transition matrices are full rank, in which case it is easy to see that we could root
the tree arbitrarily. To connect this back with biology, here we are assuming we
have sequenced each of the extant species and that moreover these sequences have
already been properly aligned. We think of the ith symbol in each of these sequences
as being an independent sample from the above model, and we want to reconstruct
the evolutionary tree that led to the species we have today as well as get some understanding of how long these evolutionary branches were. We mention as a caveat
that one of the most interesting and challenging problems in computational biology
is to perform multiple sequence alignment, and here we have assumed that a fully
aligned set of sequences is our starting point. Moreover our model for evolution is
simplistic in that we only only point mutations instead of insertions, deletions and
cross-over.
This is really two separate learning goals: Our approach for finding the topology will follow the foundational work of Steel [109] and Erdos, Steel, Szekely, and
Warnow [57]. And from this, we can apply tensor methods to find the transition
matrices following the approach of Chang [36] and later Mossel and Roch [96].
Finding the Topology
The basic idea here is to define an appropriate distance function [109] on the edges
of the tree, so that we can approximately compute the distance between leaves from
our samples and then construct the tree.
36
CHAPTER 3. TENSOR METHODS
Defining a Tree Metric
Suppose first that, for leaves a and b, we have access to the true values of F ab , where
Fijab = P(s(a) = i, s(b) = j).
In [109], Steel defined a distance metric on the tree in a way that allows us to
compute the distances between leaves a and b, given F ab . In particular, let
ψab := − ln |det(F ab )|.
Steel showed that
ψab =
X
νuv ,
(u,v)∈pab
where pab is the unique path in the tree from a to b, and




Y
Y
1
1
νuv = − ln |det(P uv )| + ln 
πu (i) − ln 
πv (i) .
2
2
i∈[k]
i∈[k]
He then showed that νuv is always non-negative (which is not obvious), and hence
ψ is indeed a metric.
The important point is that we can estimate F ab from our samples, and hence
we can (approximately) compute ψab on the leaves.
Reconstructing Quartets
Here we will use ψ to compute the topology. Fix four leaves a, b, c, and d, and
there are exactly three possible induced topologies between these leaves, given in
Figure 3.1. (Here by induced topology, we mean delete edges not on any shortest
path between any pair of the four leaves, and contract paths to a single edge if possible). Our goal is to determine which of these induced topologies is the true topology,
given the pairwise distances. Consider topology (a) on the left of Figure 3.1; in this
case, we have
ψ(a, b) + ψ(c, d) < min {ψ(a, c) + ψ(b, c), ψ(a, d) + ψ(b, d)} ,
Thus we can determine which is the true induced topology by computing three
values ψ(a, b) + ψ(c, d), ψ(a, c) + ψ(b, c), and ψ(a, d) + ψ(b, d). Whichever is the
smallest determines the induced topology because whichever nodes are paired up
are the ones with a common parent (again in the induced topology).
3.3. PHYLOGENETIC TREES AND HMMS
37
a c a b a b b d c d d c (a)
(b)
(c)
Figure 3.1: Possible quartet topologies
Indeed from just these quartet tests we can recover the topology of the tree.
For example, a pair of leaves a, b have the same parent if and only if these nodes
always have a common parent in the induced topology for each quartet test. Hence
we can pair up all of the leaves so that they have the same parent, and it is not hard
to extend this approach to recover the topology of the tree.
Handling Noise
Note that we can only approximate F ab from our samples. This translates into a
good approximation of ψab when a and b are close, but is noisy when a and b are
far away. The approach in [57] of Erdos, Steel, Szekely, and Warnow is to only use
quartets where all of the distances are short.
Finding the Transition Matrices
Here we will assume that we know the topology of the tree and T abc for all triplets
a, b, c of leaves, where
abc
Tijk
= P(s(a) = i, s(b) = j, s(c) = k).
(which we can approximate from random samples). Then consider the unique node
that lies on all of the shortest paths among a, b, and c; since we can reroot the tree
arbitrarily let this node be r. Then
X
T abc =
P(s(r) = `)P(s(a) = ·|s(r) = `) ⊗ P(s(b) = ·|s(r) = `) ⊗ P(s(c) = ·|s(r) = `)
`
=
X
P(s(r) = `)P`ra ⊗ P`rb ⊗ P`rc
`
where we have used P`rx to denote the `th row of the transition matrix P rx .
38
CHAPTER 3. TENSOR METHODS
We can now apply the algorithm in Section 3.1 to compute a tensor decomposition of T whose factors are unique up to rescaling. Furthermore the factors are
probability distributions and hence we can compute their proper normalization. We
will call this procedure a star test. (Indeed, the algorithm for tensor decompositions
in Section 3.1 has been re-discovered many times and it is also called Chang’s lemma
[36]).
In [96], Mossel and Roch use this approach to find the transition matrices of
a phylogenetic tree, given the tree topology, as follows. Let us assume that u and
v are internal nodes and that w is a leaf. Furthermore suppose that v lies on the
shortest path between u and w. The basic idea is to write
P uw = P uv P vw
and if we can find P uw and P vw (using the star tests above) then we can compute
P uv = P uw (P vw )−1 since we have assumed that the transition matrices are invertible.
However there are two serious complications:
(a) As in the case of finding the topology, long paths are very noisy.
Mossel and Roch showed that one can recover the transition matrices also using
only queries to short paths.
(b) We can only recover the tensor decomposition up to relabeling.
In the above star test, we could apply any permutation to the states of r and permute
the rows of the transition matrices P ra , P rb and P rc accordingly so that the resulting
joint distribution on a, b and c is unchanged.
However the approach of Mossel and Roch is to work instead in the framework
of PAC learning [114] where the goal is to learn a generative model that produces
almost the same joint distribution on the leaves. (In particular, if there are multiple
ways to label the internal nodes to produce the same joint distribution on the leaves,
we are indifferent to them).
Remark 3.3.1 HMMs are a special case of phylogenetic trees where the underlying
topology is a caterpillar. But note that for the above algorithm, we need that the
transition matrices and the observation matrices are full-rank.
More precisely, we require that the transition matrices are invertible and that
the observation matrices whose row space correspond to a hidden node and whose
column space correspond to the output symbols each have full row rank.
3.3. PHYLOGENETIC TREES AND HMMS
39
Beyond Full Rank?
The algorithm above assumed that all transition matrices are full rank. In fact if
we remove this assumption, then it is easy to embed an instance of the noisy parity
problem [31] which is a classic hard learning problem. Let us first define this problem
without noise:
Let S ⊂ [n], and choose Xj ∈ {0, 1}n independently
and uniformly at random,
P
for j = 1, . . . , m. Given Xj and bj = χS (Xj ) := i∈S Xj (i) mod 2 for each j, the
goal is to recover S.
This is quite easy: Let A be the matrix whose jth row is Xj and let b be a
column vector whose jth entry is bj . It is straightforward to see that 1S is a solution
to the linear system Ax = b where 1S is the indicator function for S. Furthermore if
we choose Ω(n log n) samples then A is w.h.p. full column rank and so this solution
is unique. We can then find S by solving a linear system over GF (2).
Yet a slight change in the above problem does not change the sample complexity but makes the problem drastically harder. The noisy parity problem is the
same as above but for each j we are independently given the value bj = χS (Xj ) with
probably 2/3 and otherwise bj = 1 − χS (Xj ). The challenge is that we do not know
which labels have been flipped.
Claim 3.3.2 There is a brute-force algorithm that solves the noisy parity problem
using O(n log n) samples
Proof: For each T , calculate χT (Xj )bj over the samples. Indeed χT (Xj ) and bj are
correlated if and only if S = T . This algorithm runs in time 2n (roughly). The state-of-the-art due to Blum,
Kalai, and Wasserman [31] has running time and sample complexity 2n/ log n . It is
widely believed that there is no polynomial time algorithm for noisy parity even
given any polynomial number of samples. This is an excellent example of a problem
whose sample complexity and computational complexity are (conjectured) to be wildly
different.
Next we show how to embed samples from a noisy parity problem into an
HMM, however to do so we will make use of transition matrices that are not full
rank. Consider an HMM that has n hidden nodes, where the ith hidden node
encodes is used to represent the ith coordinate of X and the running parity
X
χSi (X) :=
X(i0 ) mod 2.
i0 ≤i,i0 ∈S
40
CHAPTER 3. TENSOR METHODS
Hence each node has four possible states. We can define the following transition
matrices. Let s(i) = (xi , si ) be the state of the ith internal node where si = χSi (X).
We can define the following transition matrices:
if i + 1 ∈ S
if i + 1 ∈
/S
P
P
i+1,i
i+1,i
=
=

1

2
1
2

0

1

2
1
2

0
(0, si )
(1, si + 1 mod 2)
otherwise
(0, si )
.
(1, si )
otherwise
At each internal node we observe xi and at the last node we also observe χS (X)
with probability 2/3 and otherwise 1 − χS (X). Each sample from the noisy parity
problem is a set of observations from this HMM, and if we could learn the transition
matrices of it we would necessarily learn S and solve the noisy parity problem.
Note that here the observation matrices are certainly not full rank because
we only observe two possible emissions even though each internal node has four
possible states! Hence these problems become much harder when the transition (or
observation) matrices are not full rank!
3.4
Community Detection
Here we give applications of tensor methods to community detection. There are
many settings in which we would like to discover communities - that is, groups of
people with strong ties. Here we will focus on graph theoretic approaches, where
we will think of a community as a set of nodes that are better connected to each
other than to nodes outside of the set. There are many ways we could formalize this
notion, each of which would lead to a different optimization problem e.g. sparsest
cut or k-densest subgaph.
However each of these optimization problems is N P -hard, and even worse are
hard to approximate. Instead, we will formulate our problem in an average-case
model where there is an underlying community structure that is used to generate a
random graph, and our goal is to recover the true communities from the graph with
high probability.
3.4. COMMUNITY DETECTION
41
Block Stochastic Model
Here we introduce the block stochastic model, which is used to generate a random
graph on V with |V | = n. Additionally, the model is specified by parameters p and
q and a partitioning specified by a function π:
• π : V → [k] partitions the vertices V into k disjoint groups (we will relax this
condition later);
• Each possible edge (u, v) is chosen independently with:
(
q π(u) = π(v)
Pr[(u, v) ∈ E] =
.
p otherwise
In our setting we will set q > p, but this model has been studied in cases where
q < p too. (In particular, when q = 0 we could ask to find a k-coloring of this
random graph). Regardless, we observe a random graph generated from the above
model and our goal is to recover the partition described by π.
When is this information theoretically possible? In fact even for k = 2 where
π is a bisection, we need
r log n q−p>Ω
n
in order for the true bisection to be the uniquely smallest cut that bisects the
random graph G with high probability. If q − p is smaller, then it is not even
information theoretically possible to find π. Indeed, we should also require that
each part of the partition is large, and for simplicity we will assume that k = O(1)
and |{u|π(u) = i}| = Ω(n).
There has been a long line of work on partitioning random graphs in the block
stochastic model, culminating in the work of McSherry [91]:
Theorem 3.4.1 [91] There is an efficient algorithm that recovers π (up to relabeling) if
s
q−p
log n/δ
>c
q
qn
and succeeds with probability at least 1 − δ.
This algorithm is based on spectral clustering, where we think of the observed adjacency matrix as the sum of a rank k matrix which encodes π and an error term. If
the error is small, then we can recover something close to the true rank k matrix by
42
CHAPTER 3. TENSOR METHODS
finding the best rank k approximation to the adjacency matrix. For the full details,
see [91].
We will instead follow the approach in Anandkumar et al [9] that makes use
of tensor decompositions instead. In fact, the algorithm of [9] also works in the
mixed membership model where we allow each node to be a distribution over [k].
v
Then if π u and πP
are the probability
distributions for u and v, the probability of
P
u v
u v
an edge (u, v) is i πi πi q + i6=j πi πj p. We can interpret this probability as: u
and v choose a community according to π u and π v respectively, and if they choose
the same community there is an edge with probability q and otherwise there is an
edge with probability p.
Recall that in order to apply tensor decomposition methods what we really
need are conditionally independent random variables! In fact we will get such random variables based on counting three stars.
Counting Three Stars
We will partition V into four sets (arbitrarily) X, A, B, and C. Let Π ∈ {0, 1}V ×k
represent the (unknown) assignment of vertices to communities, such that each
row of Π contains exactly one 1. Also let R ∈ Rk×k be the matrix describing the
probability of each pair of communities having an edge. In particular,
(
q i=j
(R)ij =
.
p i 6= j
Consider the product ΠR. The ith column of ΠR encodes the probability that
an edge occurs from a vertex in community i to a given other vertex:
(ΠR)xi = Pr[(x, a) ∈ E|π(a) = i].
We will use (ΠR)A
i to denote the matrix ΠR restricted to the ith column and
the rows in A, and similarly for B and C. Moreover let pi be the fraction of nodes
in X that are in community i. Then consider the following tensor
X
X
B
C
T :=
pi Tx =
pi (ΠR)A
i ⊗ (ΠR)i ⊗ (ΠR)i .
i
i
The key claim is:
Claim 3.4.2 Let a ∈ A, b ∈ B and c ∈ C; then Ta,b,c is exactly the probability that
a random node x ∈ X is connected to a, b and c.
3.4. COMMUNITY DETECTION
43
This is immediate from the definitions above. In particular if we look at whether
(x, a), (x, b) and (x, c) are edges in G, these are conditionally independent random
variables. Then we need to prove:
2
e
(a) If |X| = Ω(|A||B||C|/
), then we can estimate T accurately
B
B
(b) The factors {(ΠR)A
i }i , {(ΠR)i }i , and {(ΠR)i }i are linearly independent, and
hence the tensor decomposition of T is unique by Theorem 3.1.3
More precisely, we need these factors to be well-conditioned so that we can approximate them from an approximation Te to T . See Section 3.2.
(c) We can recover π from {(ΠR)A
i }i up to relabeling.
Part (a) Let {Xa,b,c }a,b,c be a partition of X into almost equal sized sets, one for
each a ∈ A, b ∈ B and c ∈ C. Then
|{x ∈ Xa,b,c |(x, a), (x, b), (x, c) ∈ E}|
Tea,b,c =
|Xa,b,c |
will be close to Ta,b,c with high probability. We can then use a union bound.
Part (b) It is easy to see that R is full rank and moreover if we choose A, B and
C at random then if each community is large enough, with high probability each
community will be well-represented in A, B and C and hence the factors {(ΠR)A
i }i ,
B
B
{(ΠR)i }i , and {(ΠR)i }i will be non-negligibly far from linearly dependent.
Part (c) Note that if we have a good approximation to {(ΠR)A
i }i then we can
partition A into communities. In turn, if A is large enough then we can extend this
partitioning to the whole graph: We add a node x ∈
/ A to community i if and only if
the fraction of nodes a ∈ A with π(a) = i that x is connected to is close to q. With
high probability, this will recover the true communities.
However for a full proof of the algorithm see [9]. Anandkumar et al also give an
algorithm for mixed membership models where each πu is chosen from a Dirichlet.
We will not cover this latter extension because we will instead explain those types
of techniques in the setting of topic models next.
We note that there are powerful extensions to the block-stochastic model that
are called semi-random models. Roughly, these models allow an “adversary” to add
edges between nodes in the same cluster and delete edges between clusters after G
44
CHAPTER 3. TENSOR METHODS
is generated. If π is the best partitioning of G, then this is only more true after the
changes. Interestingly, many spectral algorithms breakdown in this more flexible
model, but there are elegant techniques for recovering π even in this more general
setting (see [60], [59]).
3.5
Extensions to Mixed Models
Here we will extend tensor spectral models to work with (some) mixed models.
Pure Topic Model
First we describe an easy application of tensor methods to pure topic models (see
[10]). Recall that there is an unknown topic matrix A and we obtain samples from
the following model:
(a) Choose topic i for document j with probability pi
(b) Choose Nj words according to the distribution Ai
If each document has at least three words, we can define the tensor Te where Tea,b,c
counts the fraction of documents in our sample whose first word, second word and
third word are a, b and c respectively. Then it is easy to see that if the number of
documents is large enough then Te converges to:
T =
r
X
pi Ai ⊗ Ai ⊗ Ai
i=1
In order to apply the algorithm in Section 3.1, we just need that A has full column
rank. In this case the factors in the decomposition are unique up to rescaling, and
the algorithm will find them. Finally, each column in A is a distribution and so
we can properly normalize these columns and compute the values pi too. Recall in
Section 3.2 we analyzed the noise tolerance of our tensor decomposition algorithm.
e and a distribution
It is easy to see that this algorithm recovers a topic matrix A
{e
pi }i that is -close to A and {pi }i respectively with high probability if we are given
at least poly(n, 1/, 1/σr ) documents of length at least three, where n is the size of
the vocabulary and σr is the smallest singular value of A.
We will refer to this as an application of tensor methods to pure models, since
each document is described by one and only one topic. Similarly, in our application to community detection, each node belonged to one and only one community.
3.5. EXTENSIONS TO MIXED MODELS
45
Finally, in our application to phylogenetic reconstruction, each hidden node was in
one and only one state. Note however that in the context of topic models, it is much
more realistic to assume that each document is itself a mixture of topics and we will
refer to these as mixed models.
Latent Dirichlet Allocation
Here we will give a tensor spectral algorithm for learning a very popular
P mixed
model, called Latent Dirichlet Allocation [30]. Let ∆ := {x ∈ Rr : x ≥ 0, i xi = 1}
denotes the r-dimensional simplex. Then we obtain samples from the following
model:
(a) Choose a mixture over topics wj ∈ ∆ for document j according to the Dirichlet
distribution Dir({αi }i )
(b) Repeat Nj times: choose a topic i from wj , and choose a word according to
the distribution Ai .
The Dirichlet distribution is defined as
Y
p(x) ∝
xαi i −1 for x ∈ ∆
i
Note that if documents are long (say Nj > n log n) then in a pure topic model, pairs
of documents often have nearly identical empirical distributions on words. But this
is no longer the case in mixed models like the one above.
The basic issue in extending our tensor spectral approach to mixed models is
that the tensor Te that counts triples of words converges to
X
T =
Dijk Ai ⊗ Aj ⊗ Ak
ijk
where Di,j,k is the probability that the first three words in a random document are
generated from topics i, j and k respectively. In a pure topic model, Di,j,k was
diagonal but for a mixed model it is not!
Definition 3.5.1 A Tucker decomposition of T is
X
T =
Di,j,k ai ⊗ bj ⊗ ck
i,j,k
where D is r1 × r2 × r3 . We call D the core tensor.
46
CHAPTER 3. TENSOR METHODS
This is different than the standard definition for a tensor decomposition where we
only summed over i = j = k. The good news is that computing a Tucker decomposition of a tensor is easy. Indeed we can always set r1 equal to the dimension of
span({Ti,∗,∗ }i ), and similarly for r2 and r3 . However the bad news is that a Tucker
decomposition is in general not unique, so even if we are given T we cannot necessarily compute the above decomposition whose factors are the topics in the topic
model.
How can we extend the tensor spectral approach to work with mixed models?
The elegant approach of Anandkumar et al [8] is based on the following idea:
Lemma 3.5.2
T =
X
Dijk Ai ⊗ Aj ⊗ Ak
ijk
S=
X
=⇒ T − S =
X
e ijk Ai ⊗ Aj ⊗ Ak
D
ijk
e ijk )Ai ⊗ Aj ⊗ Ak
(Dijk − D
ijk
Proof: The proof is a simple exercise in multilinear algebra. Hence if we have access to other tensors S which can be written using the same
factors {Ai }i in its Tucker decomposition, we can subtract T and S and hope to
make the core tensor diagonal. We can think of D as being the third order moments
of a Dirichlet distribution in our setting. What other tensors do we have access to?
Other Tensors
We described the tensor T based on the following experiment: Let Ta,b,c be the probability that the first three words in a random document are a, b and c respectively.
But we could just as well consider alternative experiments. The three experiments
we will need in order to given a tensor spectral algorithm for LDA are:
(a) Choose three documents at random, and look at the first word of each document
(b) Choose two documents at random, and look at the first two words of the first
document and the first word of the second document
(c) Choose a document at random, and look at its first three words
3.5. EXTENSIONS TO MIXED MODELS
47
These experiments result in tensors whose factors are the same, but whose cores
differ in their natural Tucker decomposition.
Definition 3.5.3 Let µ, M and D be the first, second and third order moments of
the Dirichlet distribution.
More precisely, let µi be the probability that the first word in a random document
was generated from topic i. Let Mi,j be the probability that the first and second
words in a random document are generated from topics i and j respectively. And as
before, let Di,j,k be the probability that the first three words in a random document
are generated from topics i, j and k respectively. Then letT 1 , T 2 and T 3 be the
expectation of the first, second and third experiments respectively.
P
Lemma 3.5.4 (a) T 1 = i,j,k [µ ⊗ µ ⊗ µ]i,j,k Ai ⊗ Aj ⊗ Ak
P
(b) T 2 = i,j,k [M ⊗ µ]i,j,k Ai ⊗ Aj ⊗ Ak
P
(c) T 3 = i,j,k Di,j,k Ai ⊗ Aj ⊗ Ak
Proof: Let w1 denote the first word and let t1 denote the topic of w1 (and similarly
for the other words). We can expand P[w1 = a, w2 = b, w3 = c] as:
X
P[w1 = a, w2 = b, w3 = c|t1 = i, t2 = j, t3 = k]P[t1 = i, t2 = j, t3 = k]
i,j,k
and the lemma is now immediate. 2
2
because two of the words come from the same document.
6= Ta,c,b
Note that Ta,b,c
2
2
2
Nevertheless, we can symmetrize T 2 in the natural way: Set Sa,b,c
= Ta,b,c
+ Tb,c,a
+
2
2
2
Tc,a,b . Hence Sa,b,c = Sπ(a),π(b),π(c) for any permutation π : {a, b, c} → {a, b, c}.
Our main goal is to prove the following identity:
α02 D + 2(α0 + 1)(α0 + 2)µ⊗3 − α0 (α0 + 2)M ⊗ µ(all three ways) = diag({pi }i )
P
where α0 = i αi . Hence we have that
X
α02 T 3 + 2(α0 + 1)(α0 + 2)T 1 − α0 (α0 + 2)S 2 =
pi Ai ⊗ Ai ⊗ Ai
i
The important point is that we can estimate the terms on the left hand side from our
sample (if we assume we know α0 ) and we can apply the algorithm from Section 3.1
to the tensor on the right hand side to recover the topic model, provided that A has
full column rank. In fact, we can compute α0 from our samples (see [8]) but we will
focus instead on proving the above identity.
48
CHAPTER 3. TENSOR METHODS
Moments of the Dirichlet
The main identity that we would like to establish is just a statement about the
moments of a Dirichlet distribution. In fact, we can think about the Dirichlet as
instead being defined by the following combinatorial process:
(a) Initially, there are αi balls of each color i
(b) Repeat C times: choose a ball at random, place it back with one more of its
own color
This process gives an alternative characterization of the Dirichlet distribution, from
which it is straightforward to calculate:
(a) µ = [ αα01 , αα20 , ..., ααr0 ]
( α (α +1)
i
(b) Mi,j =
(c) Ti,j,k =
i
α0 (α0 +1)
αi αj
α0 (α0 +1)
i=j
otherwise
 αi (αi +1)(αi +2)

 α0 (α0 +1)(α0 +2)
αi (αi +1)αk
α0 (α0 +1)(α0 +2)


αi αj αk
α0 (α0 +1)(α0 +2)
.
i=j=k
i = j 6= k
.
i, j, k distinct
For example for Ti,i,k this is the probability that the first two balls are color i and
the third ball is color k. The probably that the first ball is color i is αα0i and since
we place it back with one more of its own color, the probability that the second ball
k
. And the probability that the third ball is color k is αα0 +2
.
is color i as well is αα0i +1
+1
It is easy to check the above formulas in the other cases too.
Note that it is much easier to think about only the numerators in the above
formulas. If we can prove that following relation for just the numerators
D + 2µ⊗3 − M ⊗ µ(all three ways) = diag({2αi }i )
it is easy to check that we would obtain our desired formula by multiplying through
by α03 (α0 + 1)(α0 + 2).
Definition 3.5.5 Let R = num(D) + num(2µ⊗3 ) − num(M ⊗ µ)(all three ways)
Then the main lemma is:
3.5. EXTENSIONS TO MIXED MODELS
49
Lemma 3.5.6 R = diag({2αi }i )
We will establish this by a case analysis:
Claim 3.5.7 If i, j, k are distinct then Ri,j,k = 0
This is immediate since the i, j, k numerator of D, µ⊗3 and M ⊗ µ are all αi αj αk .
Claim 3.5.8 Ri,i,i = 2αi
This is also immediate since the i, i, i numerator of D is αi (αi + 1)(αi + 2) and
similarly the numerator of µ⊗3 is αi3 . Finally, the i, i, i numerator of M ⊗ µ is
αi2 (αi + 1). The case that requires some care is:
Claim 3.5.9 If i 6= k, Ri,i,k = 0
The reason this case is tricky is because the terms M ⊗ µ(all three ways) do not
all count the same. If we think of µ along the third dimension of the tensor then
the ith topic occurs twice in the same document, but if instead we think of µ as
along either the first or second dimension of the tensor, even though the ith topic
occurs twice it does not occur twice in the same document. Hence the numerator
of M ⊗ µ(all three ways) is αi (αi + 1)αk + 2αi2 αk . Also, the numerator of D is
αi (αi + 1)αk and the numerator of µ⊗3 is again αi2 αk .
These three claims together establish the above lemma. Even though the
tensor T 3 that we could immediately decompose in a pure topic model no longer
has a diagonal core tensor in a mixed model, at least in the case of LDA we can
still find a formula (each of whose terms we can estimate from our samples) that
diagonalizes the core tensor. This yields:
Theorem 3.5.10 [8] There is a polynomial time algorithm to learn a topic matrix
e that is close to the true A in a Latent Dirichlet Allocation model, provided we
A
are given at least poly(n, 1/, 1/σr , 1/αmin ) documents of length at least thee, where
n is the size of the vocabulary and σr is the smallest singular value of A and αmin
is the smallest αi .
Similarly, there are algorithms for community detection in mixed models too, where
for each node u we choose a distribution πu over clusters from a Dirichlet distribution
[9]. However these algorithms seem to be quite dependent on the assumption that
we use a Dirichlet distribution, and it seems hard to generalize these algorithms to
any other natural distributions.
50
CHAPTER 3. TENSOR METHODS
3.6
Independent Component Analysis
We can think about the tensor methods we have developed as a way to use higher
order moments to learn the parameters of a distribution (e.g. for phylogenetic
trees, HMMs, LDA, community detection) through tensor decomposition. Here we
will give another style of using the method of moments through an application to
independent component analysis which was introduced by Comon [42].
This problem is simple to define: Suppose we are given samples of the form
y = Ax + b
where we know that the variables xi are independent and the linear transformation
(A, b) is unknown. The goal is to learn A, b efficiently from a polynomial number
of samples. This problem has a long history, and the typical motivation for it is to
consider a hypothetical situation called the cocktail party problem
We have N microphones and N conversations going on in an room.
Each microphone hears a superposition of the conversations given by the
corresponding rows of A. If we think of the conversations as independent
and memoryless, can we disentangle them?
Such problems are also often referred to as blind source separation. We will follow
an approach of Frieze, Jerrum and Kannan [62].
Step 1
bx + bb so that E[b
We can always transform the problem y = Ax + b into y = Ab
xi ] = 0
2
b
bi = Ai std(xi ) where std(xi ) is
and E[b
xi ] = 1 for all i by setting b = b + A E[x] and A
the standard deviation of xi .
Note that the distribution on y has not changed, but we have put (A, x) into a
canonical form since we anyways cannot distinguish between a pair of linear transformations that have the same canonical form. So without loss of generality we have
reduced to the problem of learning
y = Ax + b
where for all i, E[xi ] = 0, E[x2i ] = 1. Also we can set b = 0 since we can easily learn
b. The crucial assumption that we will make is that A is non-singular.
3.6. INDEPENDENT COMPONENT ANALYSIS
51
Step 2
E[yy T ] = E[AxxT AT ] = AAT
The last equality follows from the condition that E[xi ] = 0, E[x2i ] = 1 and each xi is
independent. Hence we have access to M = AAT which we can learn up to arbitrary
precision by taking sufficiently many random samples. But what does this tell us
about A? We have encountered this problem before: M does not uniquely define A,
and our approach was to consider higher order tensors. This time we will proceed
in a different manner.
Since M 0 we can find B such that M = BB T . How are B and A related?
In fact, we can write
BB T = AAT ⇒ B −1 AAT (B −1 )T = I
and this implies that B −1 A is orthogonal since a square matrix times its own transpose is the identity if and only if it is orthogonal. So we have learned A up to an
unknown rotation. Can we hope to learn the rotation R = B −1 A? Hint: what if
each xi is a standard Gaussian?
In this case, Rx is also a spherical Gaussian and hence we cannot hope to
learn R without an additional assumption. In fact, this is the only case that can go
wrong: Provided the xi ’s are not Gaussian, we will be able to learn R and hence A.
For simplicity let us assume that each xi is ±1 hence E[x4i ] = 1 and yet the fourth
moment of a Gaussian is three. Note that we can apply B −1 to our samples and
hence we can imagine that we are getting samples from y = Rx. The key to our
analysis is the following functional
F (u) := E[(uT Rx)4 ]
As u ranges over the unit sphere, so does v T = uT R and so instead of minimizing F (u) over unit vectors u we can instead work with the following equivalent
optimization problem:
min E[(v T x)4 ]
kvk2 =1
What are its local minima?
"
#
X
X
T 4
E (v x) = E
(vi xi )4 + 6
(vi xi )2 (vj xj )2 =
i
=
X
i
vi4 E(x4i ) + 6
X
ij
vi2 vj2 + 3
ij
X
vi4 − 3
X
i
i
=
X
i
vi4
vi4 + 3(
X
vi2 )
i
E x4i − 3 + 3
52
CHAPTER 3. TENSOR METHODS
Hence the local minima of F (v) correspond exactly to setting vi = ±1 for some i.
Recall that v T = uT R and so this characterization implies that the local minima of
F (u) correspond to setting u to be a column of ±R.
The algorithm proceeds by using gradient descent (and a lower bound on the
Hessian) to show that you can find a local optima of F (u) quickly, and we can
then recurse on the orthogonal complement to the vector we have found to find the
other columns of R. This idea requires some care to show that the errors do not
accumulate too badly, see [62], [116], [16].
In fact what we just computed are the cumulants that are an alternative basis
for the moments of a distribution. Often these are much easier to work with since
they satisfy the appealing property that the cumulants of the sum of independent
variables Xi and Xj are the themselves the sum of the cumulants of Xi and Xj .
This is precisely the property we exploited here.
Chapter 4
Sparse Recovery
In this chapter we will study algorithms for sparse recovery: given a matrix A and
a vector b that is a sparse linear combination of its columns – i.e. Ax = b and x is
sparse – when can solve for x?
4.1
Basics
Throughout this section, we will consider only linear systems Ax = b where A has
more columns than rows. Hence there is more than one solution for x (if there is
any solution at all), and we will be interested in finding the solution that has the
smallest number of non-zeros:
Definition 4.1.1 Let kxk0 be the number of non-zero entries of x.
Unfortunately finding the sparsest solution to a system of linear equations in full
generality is computationally hard, but there will be a number of important examples
where we can solve for x efficiently.
Question 7 When can we find for the sparsest solution to Ax = b?
A trivial observation is that we can recover x when A has full column rank. In
this case we can set x = A+ b, where A+ is the left-pseudo inverse of A. Note that
this procedure works regardless of whether or not x is sparse. In contrast, when A
has more columns than rows we will need to take advantage of the sparsity of x. We
will show that under certain conditions on A, if x is sparse enough then indeed it is
the uniquely sparsest solution to Ax = b.
53
54
CHAPTER 4. SPARSE RECOVERY
Our first goal is to prove that finding the sparsest solution to a linear system
is hard. We will begin with the related problem:
Problem 1 (P) Find the sparsest non-zero vector x in a given subspace S
Khachiyan [81] proved that this problem is N P -hard, and this result has many
interesting applications that we will discuss later.
Reduction from Subset Sum
We reduce from the following variant of subset sum:
Problem 2 (S) Given
Pdistinct values α1 , . . . , αm ∈ R, does there exist a set I ⊆ [m]
such that |I| = n and i∈I αi = 0?
We will embed an instance of this problem into the problem of finding the
sparsest non-zero vector in a given subspace. We will make use of the following
mapping which is called the weird moment curve:


1
 αi 
 2 
 αi 
n
w

Γ (αi ) ⇒ 
 ...  ∈ R
 n−2 
αi 
αin
Note that this differs from the standard moment curve since the weird moment curve
has αin instead of αin−1 .
Claim 4.1.2 A set I with |I| = n has
{Γw (αi )}i∈I is linearly dependent.
P
i∈I
αi = 0 if and only if the set of vectors
Proof: Consider the determinant of the matrix whose columns are {Γw (αi )}i∈I .
Then the proof is based on the following observations:
(a) The determinant is a polynomial in the variables αi with total degree n2 + 1,
which can be seen by writing the determinant in terms of its Laplace expansion
(see e.g. [74]).
Q
(b) Moreover the determinant is divisible by i<j αi − αj , since the determinant
is zero if any αi = αj .
4.1. BASICS
55
Hence we can write the determinant as
Y
X (αi − αj )
αi
i<j
i,j∈I
i∈I
We have assumed that the αi ’s are distinct, and consequently the determinant is
zero if and only if the sum of αi = 0. We can now complete the proof that finding the sparsest non-zero vector in a
subspace is hard: We can set A to be the n × m matrix whose columns are Γw (αi ),
and let S = ker(A). Then there is a vector x ∈ S with kxk0 = n if and only if there
is a subset I with |I| = n whose corresponding submatrix is singular. If there is
no such set I then any x ∈ S has kxk0 > n. Hence if we could find the sparsest
non-zero vector in S we could solve the above variant of subset sum.
In fact, this same proof immediately yields an interesting result in computational geometry (that was “open” several years after Khachiyan’s paper).
Definition 4.1.3 A set of m vectors in Rn is in general position if every set of at
most n vectors is linearly independent.
From the above reduction we get that it is hard to decide whether a set of m
vectors in Rn is in general position or not (since there is an I with |I| = n whose
submatrix is singular if and only if the vectors Γw (αi ) are not in general position).
Now we return to our original problem:
Problem 3 (Q) Find the sparsest solution x to Ax = b
There is a subtle difference between (P) and (Q) since in (P) we restrict to non-zero
vectors x but in (Q) there is no such restriction on x. However there is a simple
many-to-one reduction from (Q) to (P).
Lemma 4.1.4 Finding the sparsest solution x to Ax = b is N P -hard.
Proof: Suppose we are given a linear system Ax = 0 and we would like to find the
sparsest non-zero solution x. Let A−i be equal to the matrix A with he ith column
deleted. Then for each i, let x−i be the sparsest solution to A−i x−i = Ai . Let i∗
be the index where x−i is the sparsest, and suppose kx−i k0 = k. We can build a
solution x to Ax = 0 with kxk0 = k + 1 by setting the i∗ th coordinate of x to be
−1. Indeed, it is not hard to see that x is the sparsest solution to Ax = 0. 56
4.2
CHAPTER 4. SPARSE RECOVERY
Uniqueness and Uncertainty Principles
Incoherence
Here we will define the notion of an incoherent matrix A, and prove that if x is
sparse enough then it is the uniquely sparsest solution to Ax = b.
Definition 4.2.1 The columns of A ∈ Rn×m are µ-incoherent if for all i 6= j:
|hAi , Aj i| ≤ µkAi k · kAj k
While the results we derive here can be extended to general A, we will restrict our
attention to the case where kAi k = 1, and hence a matrix is µ-incoherent if for all
i 6= j, |hAi , Aj i| ≤ µ.
In fact, incoherent matrices are quite common. Suppose we choose m unit
vetors at random in Rn ;qthen it is not hard to show that these vectors will be
incoherent with µ = O( lognm ). Hence even if m = n100 , these vectors will be
e √n) incoherent. In fact, there are even better constructions of incoherent
O(1/
vectors that remove the logarithmic factors; this is almost optimal since for any
m > n, any set of m vectors in Rn has incoherence at least √1n .
We will return to the following example several times: Consider the matrix
A = [I, D], where I ∈ Rn×n is the identity matrix and D ∈ Rn×n is the DFT matrix.
2π
(i−1)(j−1)
In particular, Dij = w √n
where w = ei n . This is often referred to as the
spikes-and-sines matrix. It is not hard to see that µ = √1n here.
Uncertainty Principles
The important point is that if A is incoherent, then if x is sparse enough it will be the
uniquely sparsest solution to Ax = b. These types of results were first established
by the pioneering work of Donoho and Stark [53], and are based on establishing an
uncertainty principle.
Lemma 4.2.2 Suppose we have A = [U, V ], where U and V are orthogonal. If
b = U α = V β, then kαk0 + kβk0 ≥ µ2 .
The interpretation of√this result for the spikes-and-sines matrix is that any signal
must have at least n non-zeros in the standard basis, or in the Fourier basis.
4.2. UNIQUENESS AND UNCERTAINTY PRINCIPLES
57
Informally, a signal cannot be too localized in both the time and frequency domains
simultaneously!
Proof: Since U and V are orthonormal we have that kbk2 = kαk2 = kβk2 . We
can rewrite b as either U α or V β and hence kbk22 = |β T (V T U )α|. Because A is
incoherent, we can conclude that each entry of V T U has absolute value at most
T
T
µ(A) and
p so |β (V U )α| ≤ µ(A)kαk1 kβk1 . Using Cauchy-Schwarz it follows that
kαk1 ≤ kαk0 kαk2 and thus
p
kbk22 ≤ µ(A) kαk0 kβk0 kαk2 kβk2
p
1
Rearranging, we have µ(A)
≤ kαk0 kβk0 . Finally, applying the AM-GM inequality
we get µ2 ≤ kαk0 + kβk0 and this completes the proof. Is this result tight? Indeed, returning to the spikes-and-sines example if choose
b√to be the comb√function, where the signal has equally spaced spikes at distance
n, then b has n non-zeros in the standard basis.
√ Moreover the comb function
is its own discrete Fourier transform so it also has n non-zeros when represented
using the Fourier basis.
Next, we apply the above uncertainty principle to prove a uniqueness result:
Claim 4.2.3 Suppose A = [U, V ] where U and V are orthonormal and A is µincoherent. If Ax = b and kxk0 < µ1 , then x is the uniquely sparsest solution.
Proof: Consider any alternative solution Ae
x = b. Set y = x − x
e in which case y ∈
ker(A). Write y as y = [αy , βy ]T and since Ay = 0, we have that U αy = −V βy . We
can now apply the uncertainty principle and conclude that kyk0 = kαy k0 +kβy k0 ≥ µ2 .
It is easy to see that ke
xk0 ≥ kyk0 − kxk0 > µ1 and so x
e has strictly more non-zeros
than x does, and this completes the proof. Indeed, a similar statement is true even if A is an arbitrary incoherent matrix (instead of a union of two orthonormal bases). We will discuss this extension further
in the next section.
Kruskal Rank
We can also work with a more general condition that is more powerful when proving
uniqueness; however this condition is computationally hard to verify, unlike incoherence.
58
CHAPTER 4. SPARSE RECOVERY
Definition 4.2.4 The Kruskal rank of a set of vectors {Ai }i is the maximum r such
that all subsets of r vectors are linearly independent.
In fact, we have already proven that it is N P -hard to compute the Kruskal
rank of a given set of points, since deciding whether or not the Kruskal rank is
n is precisely the problem of deciding whether the points are in general position.
Nevertheless, the Kruskal rank of A is the right parameter for analyzing how sparse
x must be in order for it to be the uniquely sparest solution to Ax = b. Suppose
the Kruskal rank of A is r.
Claim 4.2.5 If kxk0 ≤ r/2 then x is the unique sparsest solution to Ax = b.
Proof: Consider any alternative solution Ae
x = b. Again, we can write y = x − x
e in
which case y ∈ ker(A). However kyk0 ≥ r + 1 because every set of r columns of A
is linearly independent, by assumption. Then ke
xk0 ≥ kyk0 − kxk0 ≥ r/2 + 1 and so
x
e has strictly more non-zeros than x does, and this completes the proof. In fact, if A is incoherent we can lower bound its Kruskal rank (and so the
proof in the previous section can be thought of as a special case of the one in this).
Claim 4.2.6 If A is µ-incoherent then the Kruskal rank of the columns of A is at
least 1/µ.
Proof: First we note that if there is a set I of r columns of A that are linearly
dependent, then the I × I submatrix of AT A must be singular. Hence it suffices to
prove that every set I of size r, the I × I submatrix of AT A is full rank for r = 1/µ.
So consider any such a submatrix. The diagonals are one, and the off-diagonals
have absolute value at most µ by assumption. We can now apply Gershgorin’s disk
theorem and conclude that the eigenvalues of the submatrix are strictly greater than
zero provided that r ≤ 1/µ (which implies that the sum of the absolute values of
the off-diagonals in any row is strictly less than one). This completes the proof. Hence we can extend the uniqueness result in the previous section to arbitrary
incoherent matrices (instead of just ones that are the union of two orthonormal
bases). Note that this bound differs from our earlier bound by a factor of two.
Corollary 4.2.7 Suppose A is µ-incoherent. If Ax = b and kxk0 <
the uniquely sparsest solution.
1
,
2µ
then x is
There are a number of algorithms that recover x up to the uniqueness threshold in
the above corollary, and we will cover one such algorithm next.
4.3. PURSUIT ALGORITHMS
4.3
59
Pursuit Algorithms
Here we will cover algorithms for solving sparse recovery when A is incoherent. The
first such algorithm is matching pursuit and was introduced by Mallat and Zhang
[93]; we will instead analyze orthogonal matching pursuit [99]:
Orthogonal Matching Pursuit
Input: matrix A ∈ Rn×m , vector b ∈ Rn , desired number of nonzero entries k ∈ N.
Output: solution x with at most k nonzero entries.
Initialize: x0 = 0, r0 = Ax0 − b, S = ∅.
For ` = 1, 2, . . . , k
|hAj ,r`−1 i|
Choose column j that maximizes kA
.
2
j k2
Add j to S.
Set r` = projU ⊥ (b), where U = span(AS ).
If r` = 0, break.
End
Solve for xS : AS xS = b. Set xS̄ = 0.
Let A be µ-incoherent and suppose that there is a solution x with k < 1/(2µ)
nonzero entries, and hence x is the uniquely sparsest solution to the linear system.
Let T = supp(x). We will prove that orthogonal matching pursuit recovers the true
solution x. Our analysis is based on establishing the following two invariants for our
algorithm:
(a) Each index j the algorithm selects is in T .
(b) Each index j gets chosen at most once.
These two properties immediately imply that orthogonal matching pursuit
recovers the true solution x, because the residual error r` will be non-zero until S =
T , and moreover the linear system AT xT = b has a unique solution (since otherwise
x would not be the uniquely sparsest solution, which contradicts the uniqueness
property that we proved in the previous section).
Property (b) is straightforward, because once j ∈ S at every subsequent step
in the algorithm we will have that r` ⊥ U , where U = span(AS ), so hr` , Aj i = 0 if
j ∈ S. Our main goal is to establish property (a), which we will prove inductively.
The main lemma is:
Lemma 4.3.1 If S ⊆ T at the start of a stage, then the algorithm selects j ∈ T .
60
CHAPTER 4. SPARSE RECOVERY
We will first prove a helper lemma:
Lemma 4.3.2 If r`−1 is supported in T at the start of a stage, then the algorithm
selects j ∈ T .
P
Proof: Let r`−1 = i∈T yi Ai . Then we can reorder the columns of A so that the
first k 0 columns correspond to the k 0 nonzero entries of y, in decreasing order of
magnitude:
|y | ≥ |y2 | ≥ · · · ≥ |yk0 | > 0,
|1
{z
}
|yk0 +1 | = 0, |yk0 +2 | = 0, . . . , |ym | = 0.
corresponds to first k0 columns of A
where k 0 ≤ k. Hence supp(y) = {1, 2, . . . , k 0 } ⊆ T . Then to ensure that we pick
j ∈ T , a sufficient condition is that
|hA1 , r`−1 i| > |hAi , r`−1 i|
(4.1)
for all i ≥ k 0 + 1.
We can lower-bound the left-hand side of (4.1):
|hr
`−1
0
D X
k0
E
X
k
, A1 i| = |y` ||hA` , A1 i| ≥ |y1 |−|y1 |(k 0 −1)µ ≥ |y1 |(1−k 0 µ+µ),
y` A` , A1 ≥ |y1 |−
`=1
`=2
which, under the assumption that k 0 ≤ k < 1/(2µ), is strictly lower-bounded by
|y1 |(1/2 + µ).
We can then upper-bound the right-hand side of (4.1):
|hr
`−1
D k 0
k0
E
X
X
, Ai i| = y` A` , Ai ≤ |y1 |
|hA` , Ai i| ≤ |y1 |k 0 µ,
`=1
`=1
which, under the assumption that k 0 ≤ k < 1/(2µ), is strictly upper-bounded by
|y1 |/2. Since |y1 |(1/2 + µ) > |y1 |/2, we conclude that condition (4.1) holds, guaranteeing that the algorithm selects j ∈ T and this completes the proof of the lemma.
Now we can prove the main lemma:
Proof: Suppose that S ⊆ T at the start of a stage. Then the residual r`−1 is
supported in T because we can write it as
X
r`−1 = b −
zi Ai , where z = arg min kb − AS zS k2
i∈S
4.4. PRONY’S METHOD
61
Applying the above lemma, we conclude that the algorithm selects j ∈ T . This establishes property (a) inductively, and completes the proof of correctness
for orthogonal matching pursuit. Note that this algorithm works up to exactly the
threshold where we established uniqueness. However in the case where A = [U, V ]
and U and V are orthogonal, we proved a uniqueness result that is better by a factor
of two. There is no known algorithm that matches the best known uniqueness bound
there, although there are better algorithms than the one above (see e.g. [55]).
Matching Pursuit
We note that matching pursuit differs from orthogonal matching pursuit in a crucial
way: In the latter, we recompute the coefficients xi for i ∈ S at the end of each
stage because we project b perpendicular to U . However we could hope that these
coefficients do not need to be adjusted much when we add a new index j to S.
Indeed, matching pursuit does not recompute these coefficients and hence is faster
in practice, however the analysis is more involved because we need to keep track of
how the error accumulates.
4.4
Prony’s Method
The main result in this section is that any k-sparse signal can be recovered from
just the first 2k values of its discrete Fourier transform, which has the added benefit
that we can compute Ax quickly using the FFT. This is algorithm is called Prony’s
method, and dates back to 1795. This is optimal optimal relationship between the
number of rows in A and the bound on the sparsity of x; however this method is
very unstable since it involves inverting a Vandermonde matrix.
Properties of the DFT
In Prony’s method, we will make crucial use of some of the properties of the DFT.
Recall that DFT matrix has entries:
i2π(a − 1)(b − 1)
1
exp
Fa,b = √
n
n
We can write ω = ei2π/n , and then the first row is
√1 [1, ω, ω 2 , . . .], etc.
n
√1 [1, 1, . . . , 1];
n
We will make use of following basic properties of F :
the second row is
62
CHAPTER 4. SPARSE RECOVERY
(a) F is orthonormal: F H F = F F H , where F H is the Hermitian transpose of F
(b) F diagonalizes the convolution operator
In particular, we will define the convolution operation through its corresponding
linear transformation:
Definition 4.4.1 (Circulant matrix) For a vector

cn cn−1 cn−2 . . .
 c1
cn cn−1 . . .

M c =  ..
 .
cn−1 . . . . . . . . .
c = [c1 , c2 , . . . , cn ], let

c1
c2 

..  .
. 
cn
Then we can define M c x as the result of convolving c and x, denoted by c ∗ x. It is
easy to check that this coincides with the standard definition of convolution.
In fact, we can diagonalize M c using F . We will use the following fact, without
proof:
Claim 4.4.2 M c = F H diag(b
c)F , where b
c = F c.
Hence we can think about convolution as coordinate-wise multiplication in the
Fourier representation:
Corollary 4.4.3 Let z = c ∗ x; then zb = b
cx
b, where indicates coordinate-wise
multiplication.
Proof: We can write z = M c x = F H diag(b
c)F x = F H diag(b
c)b
x = F H (b
cx
b), and
this completes the proof. We introduce the following helper polynomial, in order to describe Prony’s method:
Definition 4.4.4 (Helper polynomial)
Y
p(z) =
ω −b (ω b − z)
b∈supp(x)
= 1 + λ1 z + . . . + λk z k .
Claim 4.4.5 If we know p(z), we can find supp(x).
4.4. PRONY’S METHOD
63
Proof: In fact, an index b is in the support of x if and only if p(ω b ) = 0. So we can
evaluate p at powers of ω, and the exponents where p evaluates to a non-zero are
exactly the support of x. The basic idea of Prony’s method is to use the first 2k values of the discrete Fourier
transform to find p, and hence the support of x. We can then solve a linear system
to actually find the values of x.
Finding the Helper Polynomial
Our first goal is to find the Helper polynomial. Let
v = [1, λ1 , λ2 , . . . , λk , 0, . . . , 0], and vb = F v
It is easy to see that the value of vb at index b + 1 is exactly p(ω b ).
Claim 4.4.6 supp(b
v ) = supp(x)
That is, the zeros of vb correspond roots of p, and hence non-zeros of x. Conversely,
the non-zeros of vb correspond to zeros of x. We conclude that x vb = 0, and so:
Corollary 4.4.7 M xbv = 0
Proof: We can apply Claim 4.4.2 to rewrite x vb = 0 as x
b∗v = b
0 = 0, and this
implies the corollary. Let us write out this linear system explicitly:

x
bn x
bn−1 . . . x
bn−k
 x
b
x
b
.
.
.
x
b
n
n−k+1
 1
..
..
..
 ..
 .
.
.
.

x
b
x
b
x
bk
...
x
b1
M =
 k+1
..
..
..
 ...
.
.
.

 x
b
x
b
.
.
.
x
b
2k−1
k
 2k
..
..
..
..
.
.
.
.
...
...
..
.
x
b1
x
b2
..
.
...
..
.
x
bk+2
..
.
... x
b2k+1
..
..
.
.












Recall, we do not have access to all of the entries of this matrix since we are only
given the first 2k values of the DFT of x. However consider the k × k + 1 submatrix
64
CHAPTER 4. SPARSE RECOVERY
whose top left value is x
bk+1 and whose bottom right value is x
bk . This matrix only
involves the values that we do know!
Consider



x
bk
..
.
x
bk−1
... x
b1
x
b2k−1 x
b2k−1 . . . x
bk






λ1
λ2
..
.






 = −


λk
x
bk+1
..
.
..
.
x
b2k





It turns out that this linear system is full rank, so λ is the unique solution to the
linear system (the proof is left to the reader1 ). The entries in λ are the coefficients
of p, so once we have solved for λ we can evaluate the helper polynomial on ω b to
find the support of x. All that remains is to find the values of x. Indeed, let M be
the restriction of F to the columns in S and its first 2k rows. M is a Vandermonde
matrix, so again M xS = x
b1,2,...2k has a unique solution, and we can solve this linear
system to find the non-zero values of x.
4.5
Compressed Sensing
Here we will give stable algorithms for recovering a signal x that has an almost linear
(in the number of rows of the sensing matrix) number of non-zeros. Recall that the
Kruskal rank of the columns of A is what determines how many non-zeros we can
allow in x and yet have x be the uniquely sparsest solution to Ax = b. A random
matrix has large Kruskal rank, and what we will need for compressed sensing is a
robust analogue of Kruskal rank:
Definition 4.5.1 A matrix A is RIP with constant δk if for all k-sparse vectors x
we have:
(1 − δk )kxk22 ≤ kAxk22 ≤ (1 + δk )kxk22
If A is a random m×n matrix where each entry is an independent Gaussian (N (0, 1))
then we can choose m ≈ k log n/k and δk = 1/3. Next we formalize the goals of
sparse recovery:
Definition 4.5.2 σk (x) = minw s.t.
kwk0 ≤k
kx − wk1
i.e. σk (x) is the `1 sum of all but the k largest entries of x. In particular, if kxk0 ≤ k
then σk (x) = 0.
4.5. COMPRESSED SENSING
65
Our goal is to find a w where kx − wk1 ≤ Cσk (x) from a few (Õ(k)) measurements. Note that we will note require that w is k sparse. However if x is exactly
k sparse, then any w satisfying the above condition must be exactly equal to x
and hence this new recovery goal subsumes our exact recovery goals from previous
lectures (and is indeed much stronger).
The natural (but intractable) approach is:
(P 0)
min kwk0 s.t. Aw = b
Since this is computationally hard to solve (for all A) we will work with the `1
relaxation:
(P 1)
min kwk1 s.t. Aw = b
and we will prove conditions under which the solution w to this optimization problem
satisfies w = x (or kx − wk1 ≤ Cσk (x)).
Theorem 4.5.3 [35] If δ2k + δ3k < 1 then if kxk0 ≤ k we have w = x.
Theorem 4.5.4 [34] If δ3k + 3δ4k < 2 then
C
kx − wk2 ≤ √ σk (x)
k
Note that this bounds the `2 norm of the error x − w in terms of the `1 error of the
best approximation.
Theorem 4.5.5 [40] If δ2k < 1/3 then
kx − wk1 ≤
2 + 2δ2k
σk (x)
1 − 3δ2k
We will follow the proof of [80] that greatly stream-lined the types of analysis
and made the connection between compressed sensing and almost Euclidean subsections explicit. From this viewpoint it will be much easier to draw an analogy with
error correcting codes.
Almost Euclidean Subsections
Set Γ = ker(A). We will make use of certain geometric properties of Γ (that hold
almost surely) in order to prove that basis pursuit works:
66
CHAPTER 4. SPARSE RECOVERY
Definition 4.5.6 Γ ⊆ Rn is an almost Euclidean subsection if for all v ∈ Γ,
1
C
√ kvk1 ≤ kvk2 ≤ √ kvk1
n
n
Note that the inequality √1n kvk1 ≤ kvk2 holds for all vectors, hence the second
inequality is the important part. What we are requiring is that the `1 and `2 norms
are almost equivalent after rescaling.
Question 8 If a vector v has kvk0 = o(n) then can v be in Γ?
√
No! Any such vector v would have kvk1 = o( n)kvk2 using Cauchy-Schwartz.
Let us think about these subsections geometrically. Consider the unit ball for
the `1 norm:
B1 = {v|kvk1 ≤ 1}
This is called the cross polytope and is the convex hull of the vectors {±ei }i where
ei are the standard basis vectors. Then Γ is a subspace which when intersected with
B1 results in a convex body that is close to the sphere B2 after rescaling.
In fact it has been known since the work of [63] that
p choosing Γ uniformly at
random with dim(Γ) ≥ n − m we can choose C = O( log n/m) almost surely (in
which case it is the kernel of an m × n matrix A, which will be our sensing matrix).
In the remainder of the lecture, we will establish various geometric properties of Γ
that will set the stage for compressed sensing.
Properties of Γ
Throughout this section, let S = n/C 2 .
Claim 4.5.7 Let v ∈ Γ, then either v = 0 or |supp(v)| ≥ S.
Proof:
kvk1 =
X
j∈supp(v)
|vj | ≤
p
p
C
|supp(v)| · kvk2 ≤ |supp(v)| √ kvk1
n
where the last inequality uses the property that Γ is almost Euclidean. The last
inequality implies the claim. 4.5. COMPRESSED SENSING
67
Now we can draw an analogy with error correcting codes. Recall that here we want
C ⊆ {0, 1}n . And the rate R is R = log |C|/n and the relative distance δ is
δ=
minx6=y∈C dH (x, y)
n
where dH is the Hamming distance. The goal is to find a code where R, δ = Ω(1)
and that are easy to encode and decode. In the special case of linear codes, e.g.
C = {y|y = Ax} where A is an n × Rn {0, 1}-valued matrix and x ∈ {0, 1}Rn . Then
δ=
minx6=0∈C kxk0
n
So for error correcting codes we want to find large (linear) dimensional subspaces
where each vector has a linear number of non-zeros. In compressed sensing we want
Γ to have this property too, but moreover we want that its `1 norm is also equally
spread out (e.g. most of the non-zero coordinates are large).
Definition 4.5.8 For Λ ⊆ [n], let vΛ denote the restriction of v to coordinates in
Λ. Similarly let v Λ denote the restriction of v to Λ̄.
Claim 4.5.9 Suppose v ∈ Γ and Λ ⊆ [n] and |Λ| < S/16. Then
kvΛ k1 <
kvk1
4
Proof:
kvΛ k1 ≤
p
|Λ|kvΛ k2 ≤
p
C
|Λ| √ kvk1
n
Hence not only do vectors in Γ have a linear number of non-zeros, but in fact their
`1 norm is spread out. Now we are ready to prove that (P 1) exactly recovers x when
kxk0 is sufficiently small (but nearly linear). Next lecture we will prove that it is
also stable (using the properties we have established for Γ above).
Lemma 4.5.10 Let w = x + v and v ∈ Γ where kxk0 ≤ S/16. Then kwk1 > kxk1 .
Proof: Set Λ = supp(x).
kwk1 = k(x + v)Λ k1 + k(x + v)Λ k1 = k(x + v)Λ k1 + kv Λ k1
68
CHAPTER 4. SPARSE RECOVERY
Now we can invoke triangle inequality:
kwk1 ≥ kxΛ k1 − kvΛ k1 + kv Λ k1 = kxk1 − kvΛ k1 + kv Λ k1 = xΛ k1 − 2kvΛ k1 + kvk1
However kvk1 − 2kvΛ k1 ≥ kvk1 /2 > 0 using the above claim. This implies the
lemma. Hence we can use almost Euclidean subsections to get exact sparse recovery up to
n kxk0 = S/16 = Ω(n/C 2 ) = Ω
log n/m
Next we will consider stable recovery. Our main theorem is:
Theorem 4.5.11 Let Γ = ker(A) be an almost Euclidean subspace with parameter
C. Let S = Cn2 . If Ax = Aw = b and kwk1 ≤ kxk1 we have
kx − wk1 ≤ 4 σ S (x) .
16
Proof: Let Λ ⊆ [n] be the set of S/16 coordinates maximizing kxΛ k1 . We want
to upper bound kx − wk1 . By the repeated application of the triangle inequality,
kwk1 = kwΛ k1 + kwΛ k1 ≤ kxk1 and the definition of σt (·), it follows that
kx − wk1 = k(x − w)Λ k1 + k(x − w)Λ k1
≤ k(x − w)Λ k1 + kxΛ k1 + kwΛ k1
≤ k(x − w)Λ k1 + kxΛ k1 + kxk1 − kwΛ k1
≤ 2k(x − w)Λ k1 + 2kxΛ k1
≤ 2k(x − w)Λ k1 + 2 σ S (x) .
16
Since (x − w) ∈ Γ, we can apply Claim 4.5.9 to conclude that k(x − w)Λ k1 ≤
1
kx − wk1 . Hence
4
1
kx − wk1 ≤ kx − wk1 + 2σ S (x) .
16
2
This completes the proof. Notice that in the above argument, it was the geometric properties of Γ which
played the main role. There are a number of proofs that basis pursuit works, but the
advantage of the one we presented here is that it clarifies the connection between
the classical theory of error correction over the finite fields, and the sparse recovery.
The matrix A here plays the role parity check matrix of error correcting code, and
4.5. COMPRESSED SENSING
69
hence its kernel corresponds to the codewords. There is more subtlety in the real
case though: as opposed to the finite field setting where the Hamming distance is
essentially the only reasonable way of measuring the magnitude of errors, in the
real case there is an interplay among many different norms, giving rise to some
phenomenon not present in the finite field case.
In fact, one of the central open question of the field is to give a deterministic
construction of RIP matrices:
Question 9 (Open) Is there an explicit construction of RIP matrices, or equivalently an almost Euclidean subsection Γ?
In contrast, there are many explicit constructions of asymptotically good codes. The
best known deterministic construction is due to Guruswami, Lee and Razborov:
Theorem 4.5.12 [69] There is a polynomial time deterministic algorithm for constructing an almost Euclidean subspace Γ with parameter C ∼ (log n)log log log n
We note that it is easy to achieve weaker guarantees, such as ∀0 6= v ∈ Γ, supp(v) =
Ω(n), but these do not suffice for compressed sensing since we also require that the
`1 weight is spread out too.
70
CHAPTER 4. SPARSE RECOVERY
Chapter 5
Dictionary Learning
In this chapter we will study dictionary learning, where we are given many examples
b1 , b2 , ..., bp and we would like to find a dictionary A so that every example can be
written as a sparse linear combination of the columns in A.
5.1
Background
In the previous chapter, we studied algorithms for sparse recovery. However in many
applications, the dictionary A is unknown and has to be learned from examples. An
alternative motivation is that often one hand designs families of features to serve as
an appropriate dictionary, e.g. sinusoids, wavelets, ridgelets, curvelets, etc. [94] and
each one is useful in different settings: wavelets for impulsive events, ridgelets for
discontinuities in edges, curvelets for smooth curves, etc. But given a new collection
of data, can we learn the right families to represent the signal automatically?
Recall that it is N P -hard to find the sparsest solution to an arbitrary linear
system. However there are important classes of problems for which the problem
can be solved efficiently. Similarly, we cannot hope to solve the dictionary learning
problem for an arbitrary dictionary. We will instead focus on designing algorithms
for dictionary learning in the settings where we already know that we can solve the
corresponding sparse recovery problem too. After all, finding a dictionary is only
useful if you can use it. The three most important cases where we can do sparse
recovery are:
(a) A has full column rank
(b) A is incoherent
71
72
CHAPTER 5. DICTIONARY LEARNING
(c) A is RIP
We will present an algorithm of Spielman, Wang and Wright [108] that succeeds
(under reasonable distributional assumptions) if A is full rank, and if each bi is a
e √n) columns in A. Next, we will give an algorithm
linear combination of at most O(
of Arora, Ge and Moitra [15] for the incoherent and overcomplete case that also
e √n) sparsity. We note that Agarwal et al [2], [3] recently and
succeeds up to O(
independently gave alternative algorithms for dictionary learning that work up to a
e 1/4 ).
weaker sparsity bound of O(n
History
The dictionary learning problem was introduced by Olshausen and Field [97], who
were interested in understanding various properties of the mammalian visual cortex. Neuroscientists often measure the receptive field of neurons – namely, how the
neurons respond to various types of stimuli. Indeed, their response is well-known
to be localized, bandpass and oriented and Olshausen and Field observed that if one
learns a dictionary for a collection of natural images, the elements of the dictionary
often have many of these same properties. Their work suggested that an important
step in understanding the visual system could be in identifying the basis it uses to
represent natural images.
Dictionary learning, or as it is often called sparse coding, is a basic building
block of many machine learning systems. This algorithmic primitive arises in applications ranging from denoising, edge-detection, super-resolution and compression.
Dictionary learning is also used in the design of some deep learning architectures.
Popular approaches to solving this problem in practice are variants of the standard
alternating minimization approach. Suppose the pairs (xi , bi ) are collected into the
columns of matrices X and B respectively. Then our goal is to compute A and X
from B in such a way that the columns of X are sparse.
Method of Optimal Directions [56] : Start with an initial guess for A, and
then alternately update either A or X:
• Given A, compute a sparse X so that AX ≈ B (using e.g. matching pursuit
[93] or basis pursuit [39])
• Given X, compute the A that minimizes kAX − BkF
This algorithm converges to a local optimum, because in each step the error kAX −
Y kF will only decrease.
5.2. FULL RANK DICTIONARIES
73
K-SVD [5] Start with an initial guess for A. Then repeat the following procedure:
• Given A, compute a sparse X so that AX ≈ B (again, using a pursuit method)
• Group all data points B (1) where the corresponding X vector has a non-zero
at index i. Subtract off components in the other directions
X
(1)
B (1) −
Aj Xj
j6=i
• Compute the first singular vector v1 of the residual matrix, and update the
column Ai to v1
In practice, these algorithms are highly sensitive to their starting conditions. In
the next sections, we will present alternative approaches to dictionary learning with
provable guarantees. Recall that some dictionaries are easy to solve sparse recovery
problems for, and some are hard. Then an important rationale for preferring provable algorithms here is that if the examples are generated from an easy dictionary,
our algorithms will really learn an easy dictionary (but for the above heuristics this
need not be the case).
5.2
Full Rank Dictionaries
Here we present a recent algorithm of Spielman, Wang and Wright [108] that works
e √n) non-zeros, under certain distribuwhen A has full column rank and X has O(
tional conditions. First we define the model:
The Model
The distributional model in [108] is in fact more general than the one we will work
with here; we do this merely to simplify the analysis and notation but the proof will
be nearly identical. Let θ ∈ [ n1 , √n 1log n ] be the sparsity parameter. Then the vectors
xi are generated according to the following procedure:
(a) Each coordinate of xi is chosen (independently) to be nonzero with probability
θ
(b) If the jth coordinate is non-zero, then its value is sampled (independently)
from a standard Gaussian N (0, 1).
74
CHAPTER 5. DICTIONARY LEARNING
We observe samples bi = Axi . The dictionary A is an unknown invertible n × n
matrix, and we are asked to recover A exactly from a polynomial number of samples
of the form bi .
Theorem 5.2.1 [108] There is a polynomial time algorithm to learn a full-rank
dictionary A exactly given a polynomial number of samples from the above model.
Strictly speaking, if the coordinates of xi are perfectly independent then we
could recover A alternatively using provable algorithms for independent component
analysis [62], but the dictionary learning algorithms work more generally even when
the coordinates are not independent.
Outline
The basic idea is to consider the row space of B which we will denote by R = {wT B}.
Note that A−1 B = X and hence the rows of X are contained in R. The crucial
observation is:
Observation 5.2.2 The rows of X are the sparsest non-zero vectors in R.
Of course finding the sparsest non-zero vector in a subspace is hard, so we
will instead consider an `1 -relaxation similar to what we did for sparse recovery.
Consider the following four optimization problems:
(P 0)
min kwT Bk0 s.t. w 6= 0
This optimization problem asks for the sparsest (non-zero) vector in R. However it
is N P -hard. We instead consider:
(P 1)
min kwT Bk1 s.t. rT w = 1
The constraint rT w fixes a normalization, and we will explain how to choose r
later. The above optimization problem can be solve efficiently because it is a linear
program. We would like to show that its optimal solution wT B is a (scaled) row of
X. In fact we can transform the above linear program into a simpler one that will
be easier to analyze:
(Q1)
min kz T Xk1 s.t. cT z = 1
There is a bijection between the solutions of this linear program and those of (P 1)
given by z = AT w and c = A−1 r. Let us set r to be a column of B and hence c is
the corresponding column of X and is sparse. Now we can explain the intuition:
5.2. FULL RANK DICTIONARIES
75
• We will prove that the solution to (Q1) is sparse, in particular supp(z) ⊆
supp(c).
• And for sparse z we will have that kz T Xk1 ≈ kzk1 (after an appropriate
scaling). Hence we can instead analyze the linear program:
(Q10 )
min kzk1 s.t. cT z = 1
Note that |supp(z)| = 1 if c has a coordinate that is strictly the largest in
absolute value. Recall that we chose c to be a column of X, and in our distributional
model the non zeros in X are Gaussian. Hence almost surely c will have a strictly
largest coordinate ci in absolute value, and if we solve (P 1) the vector in the objective
function (namely wT B) will be the ith row of X up to rescaling.
• Lastly we can repeatedly solve (P 1) and find all of the rows of X and after
correcting their scaling we can solve for A by computing A = BX + .
Step 1
Suppose that z∗ is an optimal solution to (Q1), where c = xi is a column of X.
Set J = supp(c), we can write z∗ = z0 + z1 where z0 is supported in J and z1 is
supported in J. Note that cT z0 = cT z∗ . We would like to show that z0 is at least as
good of a solution to (Q1) as z∗ is. In particular we want to prove that
kz0T Xk1 < kz∗T Xk1
Definition 5.2.3 If R is a set of rows and C is a set of columns, let XCR be the
submatrix of X that is the intersection of those rows and columns.
Let S be the set of columns of X that have a non-zero entry in J. That is
S = {j|XjJ 6= ~0}. We now compute:
kz∗T Xk1 = kz∗T XS k1 + kz∗T XS k1
≥ kz0T XS k1 − kz1T XS k1 + kz1T XS k1
≥ kz0T Xk1 − 2kz1T XS k1 + kz1T Xk1
It remains to show that
(5.1)
kz1T Xk1 − 2kz1T XS k1 > 0
76
CHAPTER 5. DICTIONARY LEARNING
Let us suppose that z1 is fixed (we can apply a union bound over an ε-net of this
space to complete the argument). Then S is a random set and we can compute:
E[kz1T XS k1 ] =
|S|
E[kz1T Xk1 ]
p
The expected size of S is p × E[|supp(xi )|] × θ = θ2 np = o(p). Together, these imply
that
2 E[|S|]
T
T
E[kz1T Xk1 ]
E[kz1 Xk1 − 2kz1 XS k1 ] = 1 −
p
is bounded away from zero, and thus (5.1) holds with high probability for any fixed
z1 . We can take a union bound over an ε-net of all possible z1 ’s and conclude that
(5.1) holds for all z1 ’s. This in turn implies that if z1 is non-zero then kz0T Xk1 <
kz∗T Xk1 but this is a contradiction since we assumed that z∗ is an optimal solution
to (Q1). We conclude that z1 is zero and so supp(z∗ ) ⊆ supp(c), as desired.
Step 2
We wish to show that:
kz T Xk1 ≈ kz T k1
up to some scaling factor provided that supp(z) is small enough. Recall from the
e
previous step we know that supp(z) ⊆ supp(c) = J and |J| ≤ O(θn).
Note that the
J
2
typical column of X has θ|J| = θ n = o(1) nonzero entries in expectation. That
means that, with high probability, the vast majority of the columns in X J have at
most one non-zero entry. It is easy to see that:
Claim 5.2.4 If each column of X J has at most one non-zero entry, then
E[kzJT X J k1 ] = C
p
kzJ k1
|J|
where C is the expected absolute value of a non-zero in X.
So we can establish Step 2 by bounding the contribution of columns of X J
with two or more non-zeros. Hence this completes
the proof. (The more√concise
√
description of this step is that since |J| is n and each column of X has n nonzeros, the matrix X J acts like a scaled identity matrix).
5.3. OVERCOMPLETE DICTIONARIES
77
Step 3
We can now put everything together. Since c = xi , when we solve (P 1) we will get
the ith row of X up to scaling. If we solve (P 1) repeatedly, we will find all rows of
X (and can delete duplicates since now two rows of X will be scaled copies of each
other).
Finally we can compute the correct scaling (up to sign) e.g. by using the
assumption that non-zero entries are distributed as N (0, 1). Hence we can solve for
X up to flipping the sign of its rows, and if p is large enough (i.e. if we take enough
samples) then X will have a left pseudo-inverse and we can compute A = BX +
which will recover A up to a permutation and flipping the signs of its columns
(which is the best we could hope for).
5.3
Overcomplete Dictionaries
Here we will present a recent algorithm of Arora, Ge and Moitra [15] that works for
incoherent and overcomplete dictionaries. The crucial idea behind the algorithm is
a connection to an overlapping clustering problem. We will consider a model that
is similar to that in [108]. Let k be the sparsity parameter:
(a) The support of xi is chosen uniformly at random from all size k subsets of [m]
(b) If the jth coordinate is non-zero, then its value is independently chosen to be
+1 or −1 (with equal probability)
The main result in [15] is:
Theorem 5.3.1 [15] There is √
a polynomial time algorithm to learn A exactly if A
is µ-incoherent and k ≤ c min( n/µ log n, m1/2−η ) if we are given samples from the
above model. The running time and sample complexity are poly(n, m).
Recall that methods like K-SVD rely on the intuition that if we have the true
dictionary A, we can find X and if we have X we can find a good approximation
to A. However the trouble in analyzing these approaches is that they start from a
dictionary that is very far from the true one, so how do these algorithms still make
progress? The basic idea of the algorithm in [15] is to break the cycle by first finding
the support of X without knowing the dictionary A.
Our first step is to build a graph which we will call the intersection graph:
78
CHAPTER 5. DICTIONARY LEARNING
Definition 5.3.2 Given p samples b1 , b2 , ..., bp the intersection graph G is a graph
on V = {b1 , b2 , ..., bp } where (bi , bj ) is an edge if and only if |hbi , bj i| > τ
How should we choose τ ? We want the following properties to hold. Throughout this section we will let Si denote the support of xi .
(a) If (i, j) is an edge then Si ∩ Sj 6= ∅
(b) If Si ∩ Sj 6= ∅ then the probability that (i, j) is an edge is at least
1
2
Given some row in X we can think of all of the examples where this row is non-zero
as belonging to the same community. Then the interpretation of the above graph
is that if there is an edge between (i, j) we want that i and j necessarily belong to
the same community. Moreover if (i, j) do indeed belong to the same community, it
should be reasonably likely that there is also an edge.
We can directly compute the above graph given our examples and the basic
idea is that we can hope to learn the support of X by finding the communities. The
key point is that this departs from standard clustering problems precisely because
each sample xi has k non-zeros and so each node belongs to polynomially many
communities so what we are looking for is an overlapping clustering.
Intersection Graph
Here we choose how to set τ so that the properties above hold with high probability.
We can bound
|hbi , bj i − hxi , xj i| ≤ |Si ||Sj |µ = k 2 µ
and hence if k 2 µ < 12 and we choose τ = 12 we certainly satisfy condition (a) above.
Moreover it is not hard to see that if Si and Sj intersect then the probability that
1
hxi , xj i is non-zero
√ is at least 2 as well and this yields condition (b). However since
µ is roughly 1/ n for random dictionaries (ignoring poly-logarithmic factors), we
would need k < n1/4 which is a much more stringent condition than we would need
to solve the sparse recovery problem.
The above analysis is wasteful in that it does not make use of the random
signs of the non-zeros in X. We will instead appeal to the following concentration
inequality:
Lemma 5.3.3 (Hanson-Wright) Let x be a vector whose components are independent sub-Gaussian random variables, which satisfy E[Xi ] = 0 and V ar[Xi ] = 1.
5.3. OVERCOMPLETE DICTIONARIES
79
Let M be a symmetric n × n matrix. Then, for every t ≥ 0 we have:
2
t
t
T
P[|x M x − trace(M )| > t] ≤ 2 exp −c min
,
kM k2F kM k2
Let Si and Sj be disjoint. Set N = (AT A)SSij and
M=
0
1 T
N
2
1
N
2
0
,
and let y be the vector which is the concatenation of xi restricted to Si and xj
restricted to Sj . Then y T M y = hbi , bj i and we can appeal to the above lemma to
bound the deviation of hbi , bj i from its expectation. In particular, trace(M ) = 0 and
kM k2F ≤ µ2 k 2 which implies kM k2 ≤ µk.
1
Hence if k ≤ µ log
then with high probability we have that |hbi , bj i| ≤ 13 . An
n
identical argument works when Si and Sj intersect (where instead we zero out the
entries in N that correspond to the same column in A). So if we make use of the
randomness in the signs of X the intersection graph (for τ = 21 ) satisfies our desired
properties provided that k is a log n factor smaller than what we would need to solve
sparse recovery even if we knew A.
Community Finding
Consider the communities Cj = {bi |Si 3 j}. Then for each pair b1 , b2 ∈ Cj there is
an edge (b1 , b2 ) with probability at least 12 , and moreover our intersection graph can
be covered by m dense communities {Cj }j . We will introduce the basic approach
through the following thought experiment:
Suppose we find b1 , b2 and b3 that are a triangle in G. We know that
S1 ∩ S2 , S1 ∩ S3 and S2 ∩ S3 are each non-empty but how do we decide
if S1 ∩ S2 ∩ S3 is non-empty too?
Alternatively, given three nodes where each pair belong to the same community,
how do we know whether or not all three belong to one community? The intuition
is that if b1 , b2 and b3 all belong to a common community then it is more likely that
a random new node b4 is connected to all of them than if they don’t!
What we need is a lower bound on the probability that b4 connects to each
of b1 , b2 and b3 in the case where S1 ∩ S2 ∩ S3 is non-empty, and we need an upper
bound when the intersection is empty. It is easy to see that:
80
CHAPTER 5. DICTIONARY LEARNING
Claim 5.3.4 If S1 ∩S2 ∩S3 6= ∅ then (b4 , bi ) is an edge for i = 1, 2, 3 with probability
at least Ω(k/m)
This claim is true because if i ∈ S1 ∩ S2 ∩ S3 , then the probability that S4
contains i is k/m. Next we prove the upper bound. Let a = |S1 ∩ S2 |, b = |S1 ∩ S3 |
and c = |S2 ∩ S3 |. Then:
Lemma 5.3.5 If S1 ∩S2 ∩S3 = ∅ the probability that (b4 , bi ) is an edge for i = 1, 2, 3
is at most
6
k 3 (a + b + c)
k
+
O
m3
m2
Proof: We know that if (b4 , bi ) is an edge for i = 1, 2, 3 then S4 ∩ Si must be
non-empty for i = 1, 2, 3. We can break up this event into subcases:
(a) Either S4 intersects S1 , S2 and S3 disjointly (i.e. it does not contain any index
that is in more than one of he other sets)
(b) Or there is some pair i 6= j ∈ {1, 2, 3} so that Si ∩ Sj intersects S4 and S4
intersects the remaining set in another index
The probability of (a) is at most the probability that |S4 ∩ (S1 ∪ S2 ∪ S3 )| ≥ 3 which
k3
is at most (3k)3 m
3 . Similarly the probability of (b) for (say) i = 1, j = 2 is at most
ak3
. This implies the lemma. m2
We need the lower bound in Claim 5.3.4 to be asymptotically smaller than the
upper bound in Lemma 5.3.5. It is easy to see that with high probability for any
i, j, |Si ∩ Sj | = O(1) and hence we want (roughly)
k
k6
k3
>> 3 + 2
m
m
m
which is true if k < m2/5 . When this holds, for every triple b1 , b2 , b3 we will be able
to determine whether or not S1 ∩ S2 ∩ S3 is empty with high probability by counting
how many other nodes b4 have an edge to all of them.
Now we are ready to give an algorithm for finding the communities.
5.3. OVERCOMPLETE DICTIONARIES
81
CommunityFinding [15]
Input: intersection graph G
Output: communities {Cj }
For each edge (b1 , b2 )
Set C1,2 = {b3 |S1 ∩ S2 ∩ S3 6= ∅} ∪ {b1 , b2 }
End
Remove sets C1,2 that strictly contain another set Ci,j
Definition 5.3.6 If S1 ∩ S2 = {j} we will call the pair (b1 , b2 ) an identifying pair
for community j
It is easy to see that with high probability each community will have an identifying
pair if the number of samples p is large enough. Then consider
C1,2 = {b3 |S1 ∩ S2 ∩ S3 6= ∅} ∪ {b1 , b2 }
If (b1 , b2 ) is an identifying pair for community j then the above set C1,2 is exactly Cj
(and we can compute this set by using the above test for deciding whether or not
S1 ∩ S2 ∩ S3 is empty).
Moreover if (b1 , b2 ) is not an identifying pair but rather S1 ∩ S2 has more than
one element we would have instead C1,2 = ∪j∈S1 ∩S2 Cj in which case the set C1,2 will
be deleted in the last step. This
√ algorithm outputs the correct communities with
high probability if k ≤ c min( n/µ log n, m2/5 ). In [15] the authors give a higherorder algorithm for community finding that works when k ≤ m1/2−η for any η > 0,
however the running time is a polynomial whose exponent depends on η.
All that remains to recover the true dictionary is to partition the community
Cj into those where the jth coordinate is +1 and those where it is −1. In fact if
S1 ∩ S2 = {j} then the sign of hb1 , b2 i tells us whether or not their jth coordinate
has the same sign or a different sign. It is not hard to show that there are enough
such pairs in a typical community that we can successfully partition Cj into two sets
where all the examples in one have their jth coordinate equal to +1 and all those
in the other are −1 (of course we do not know which is which). This in turn allows
us to compute X up to a permutation or flipping the signs of its rows, and again we
can set A = BX + and compute the true dictionary exactly.
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CHAPTER 5. DICTIONARY LEARNING
Chapter 6
Gaussian Mixture Models
In this chapter we will study Gaussian mixture models and clustering. The basic
problem is, given random samples from a mixture of k Gaussians, we would like
to give an efficient algorithm to learn its parameters using few samples. If these
parameters are accurate, we can then cluster the samples and our error will be
nearly as accurate as the Bayes optimal classifier.
6.1
History
The problem of learning the parameters of a mixture of Gaussians dates back to the
famous statistician Karl Pearson (1894) who was interested in biology and evolution.
In fact, there was a particular species of crab called the Naples crab that inhabited
the region around him. He took thousands of samples from this population and
measured some physical characteristic of each sample. He plotted the frequency
of occurrence, but the resulting density function surprised him. He expected that
it would be Gaussian, but in fact it was not even symmetric around its maximum
value. See Figure 6.1. He hypothesized that maybe the Naples crab was not one
species but rather two, and that the density function he observed could be explained
as a mixture of Gaussians.
In this remarkable study Pearson introduced the method of moments. His basic
idea was to compute empirical moments from his samples, and use each of these
empirical moments to set up a system of polynomial equations on the parameters
of the mixture. He solved this system by hand! In fact, we will return to his basic
approach later in this unit.
83
84
CHAPTER 6. GAUSSIAN MIXTURE MODELS
Basics
Here we formally describe the problem of learning mixtures of Gaussians. Recall
that for a univariate Gaussian we have that its density function is given by:
2
N (µ, σ ) = √
1
2πσ 2
exp
−(x − µ)2
2σ 2
The density of a multidimensional Gaussian in Rn is given by:
1
N (µ, Σ) =
exp
n/2
(2π) det(Σ)1/2
−(x − µ)> Σ−1 (x − µ)
2
Here Σ is the covariance matrix. If Σ = In and µ = ~0 then the distribution is just:
N (0, 1) × N (0, 1) × ... × N (0, 1).
A mixture of two Gaussians is a distribution whose density function is:
F (x) = w1 F1 (x) + (1 − w1 )F2 (x)
where F1 and F2 are Gaussians. We can generate a random sample as follows: with
probability w1 we output a random sample from F1 , and otherwise we output a
random sample from F2 . Our basic problem is to learn the parameters that describe
the mixture given random samples from F . We note that we will measure how good
an algorithm is by both its sample complexity and its running time.
Method of Moments
Pearson used the method of moments to fit a mixture of two Gaussians to his data.
The moments of a mixture of Gaussians are themselves a polynomial in the unknown
parameters, which we will denote by Mr .
E
x←F1 (x)
[xr ] = Mr (µ, σ 2 )
Then we can write
E
x←F (x)
[xr ] = w1 Mr (µ1 , σ12 ) + (1 − w1 )Mr (µ2 , σ22 ) = Pr (w1 , µ1 , σ1 , µ22 , σ22 )
And hence the rth raw moment of a mixture of two Gaussians is itself a degree r + 1
polynomial (Pr ) in the unknown parameters.
85
0
5
10
15
20
6.1. HISTORY
0.58
0.60
0.62
0.64
0.66
0.68
0.70
Figure 6.1: A fit of a mixture of two univariate Gaussians to the Pearson’s data on
Naples crabs, created by Peter Macdonald using R
Pearson’s Sixth Moment Test: We can estimate Ex←F [xr ] from random samples: Let S be our set of samples. Then we can compute:
X
fr = 1
M
xr
|S| x∈S
fr will be additively
And given a polynomial number of samples (for any r = O(1)), M
close to Ex←F (x) [xr ]. Pearson’s approach was:
• Set up a system of polynomial equations
n
o
fr , r = 1, 2, ...5
Pr (w1 , µ1 , σ1 , µ22 , σ22 ) = M
• Solve this system. Each solution is a setting of all five parameters that explains
the first five empirical moments.
Pearson solved the above system of polynomial equations by hand, and he
found a number of candidate solutions. Each solution corresponds to a simultaneous
setting of the parameters so that the moments of the mixture would match the
empirical moments. But how can we choose among these candidate solutions? Some
of the solutions were clearly not right; some had negative values for the variance,
or a value for the mixing weight not in the range [0, 1]. But even after eliminating
these solutions, Pearson was still left with more than one candidate. His approach
was to choose the root whose prediction is closest to the empirical sixth moment
f6 . This is called the sixth moment test.
M
86
CHAPTER 6. GAUSSIAN MIXTURE MODELS
Expectation-Maximization
Much of modern statistics instead focuses on the maximum likelihood estimator,
which would choose to set the parameters to as to maximize the probability that
the mixture would generate the observed samples. Unfortunately, this estimator
is N P -hard to compute [18]. The popular alternative is known as expectationmaximization and was introduced in a deeply influential paper of Dempster, Laird,
Rubin [50]. The basic approach is to repeat the following steps until convergence:
• For each x ∈ S, calculate the posterior probability:
w1 (x) =
w1 F1 (x)
w1 F1 (x) + (1 − w1 )F2 (x)
• Update the mixing weights:
P
w1 ←
w1 (x)
|S|
x∈S
• Re-estimate the parameters:
P
P
wi (x)(x − µi )(x − µi )>
x∈S wi (x)x
µi ← P
, Σi ← x∈S P
x∈S wi (x)
x∈S wi (x)
This approach gets stuck in local optima, and is in general quite sensitive to how it
is initialized (see e.g. [105]).
6.2
Clustering-Based Algorithms
Our basic goal will be to give algorithms that provably compute the true parameters of a mixture of Gaussians, given a polynomial number of random samples. This
question was introduced in the seminal paper of Dasgupta [45], and the first generation of algorithms focused on the case where the components of the mixture have
essentially no “overlap”. The next generation algorithms are based on algebraic
ideas, and avoid clustering altogether.
Before we proceed, we will discuss some of the counter-intuitive properties of
high-dimensional Gaussians. To simplify the discussion, we will focus on spherical
Gaussians N (µ, σ 2 I) in Rn .
Fact 6.2.1 The maximum value of the density function is at x = µ.
6.2. CLUSTERING-BASED ALGORITHMS
87
2
2
Fact
√ 6.2.2 Almost all of the weight of the density function has kx − µk2 = σ n ±
2
σ n log n
These facts seem to be inconsistent, but the explanation is that the surface area
increases faster as the radius R increases than the value of the density function
decreases, until we reach R2 ≈ σ 2 n. Hence we should
think about a high-dimensional
√
spherical Gaussian as being a ball of radius σ n with a thin shell.
e √n) Separation
Dasgupta [45] – Ω(
Dasgupta gave the first provable
algorithms for learning mixtures of Gaussians, and
e √nσmax ) where σmax is the maximum variance of any
required that kµi − µj k2 ≥ Ω(
Gaussian in any direction (e.g. if the components are not spherical). Note that the
constant in the separation depends on wmin , and we assume we know this parameter
(or a lower bound on it).
The basic idea behind the algorithm is to project the mixture onto log k dimensions uniformly at random. This projection will preserve distances between each
pair of centers µi and µj with high probability, but will contract distances between
samples from the same component and make each component closer to spherical,
thus making it easier to cluster. We can then cluster all of the samples into which
component generated them, and then for each cluster we can choose the empirical
mean and empirical covariance which will with high probability be a good estimate
of µi and Σi . Additionally we can estimate wi by how large each cluster is.
Informally, we can think of this separation condition as: if we think of each
Gaussian as a spherical ball, then if the components are far enough apart then these
balls will be disjoint.
e 1/4 ) Separation
Arora and Kannan [18], Dasgupta and Schulman [53] – Ω(n
√
We will describe the approach in [18] in detail. The basic question is, if n separation is the threshold when we can think of the components as disjoint, then how can
we learn when the components are much closer? In fact, even if the components are
e 1/4 ) separated then it is still true that every pair of samples from the same
only Ω(n
component is closer than every pair of samples from different components. How can
this be? The explanation is that even though the balls representing each component
are no longer disjoint, we are still very unlikely to sample from their overlap region.
Consider x, x0 ← F1 and y ← F2 .
88
CHAPTER 6. GAUSSIAN MIXTURE MODELS
Claim 6.2.3 All of the vectors x − µ1 , x0 − µ1 , µ1 − µ2 , y − µ2 are nearly orthogonal
(whp)
This claim is immediate since the vectors x − µ1 , x0 − µ1 , y − µ2 are uniform from a
sphere, and µ1 − µ2 is the only fixed vector. In fact, any set of vectors in which all
but one is uniformly random from a sphere are nearly orthogonal.
Now we can compute:
kx − x0 k2 ≈ kx − µ1 k2 + kµ1 − x0 k2
p
≈ 2nσ 2 ± 2σ 2 n log n
And similarly:
kx − yk2 ≈ kx − µ1 k2 + kµ1 − µ2 k2 + kµ2 − yk2
p
≈ 2nσ 2 + kµ1 − µ2 k2 ± 2σ 2 n log n
e 1/4 , σ) then kµ1 −µ2 k2 is larger than the error term and each
Hence if kµ1 −µ2 k = Ω(n
pair of samples from the same component will be closer than each pair from different
components. Indeed we can find the right threshold τ and correctly cluster all of
the samples. Again, we can output the empirical mean, empirical covariance and
relative size of each cluster and these will be good estimates of the true parameters.
e 1/4 ) Separation
Vempala and Wang [117] – Ω(k
Vempala and Wang [117] removed the dependence on n, and replaced it with a
separation condition that depends on k – the number of components. The idea is
that if we could project the mixture into the subspace T spanned by {µ1 , . . . , µk },
we would preserve the separation between each pair of components but reduce the
ambient dimension.
So how can we find T , the subspace spanned by the means? We will restrict
our discussion to a mixture of spherical Gaussians with a common variance σ 2 I. Let
x ∼ F be a random sample from the mixture, then we can write x = c + z where
z ∼ N (0, σ 2 In ) and c is a random vector that takes the value µi with probability wi
for each i ∈ [k]. So:
T
T
T
E[xx ] = E[cc ] + E[zz ] =
k
X
2
w i µi µ>
i + σ In
i=1
Hence the top left singular vectors of E[xxT ] whose singular value is strictly larger
than σ 2 exactly span T . We can then estimate E[xxT ] from sufficiently many random
samples, compute its singular value decomposition and project the mixture onto T
and invoke the algorithm of [18].
6.3. DISCUSSION OF DENSITY ESTIMATION
89
Brubaker and Vempala [32] – Separating Hyperplane
What if the largest variance of any component is much larger than the separation
between the components? Brubaker and Vempala [32] observed that none of the
existing algorithms succeed for the parallel pancakes example, depicted in Figure ??
even though there is a hyperplane that separates the mixture so that almost all
of one component is on one side, and almost all of the other component is on the
other side. [32] gave an algorithm that succeeds, provided there is such a separating
hyperplane, however the conditions are more complex to state for mixtures of more
than two Gaussians. Note that not all mixtures that we could hope to learn have
such a separating hyperplane. See e.g. Figure ??.
6.3
Discussion of Density Estimation
The algorithms we have discussed so far [45], [53], [18], [117], [1], [32] have focused
on clustering; can we give efficient learning algorithms even when clustering is impossible? Consider a mixture of two Gaussians F = w1 F1 + w2 F2 . The separation
conditions we have considered so far each imply that dT V (F1 , F2 ) = 1 − o(1). In
particular, the components have negligible overlap. However if dT V (F1 , F2 ) = 1/2
we cannot hope to learn which component generated each sample.
More precisely, the total variation distance between two distributions F and
G measures how well we can couple them:
Definition 6.3.1 A coupling between F and G is a distribution on pairs (x, y) so
that the marginal distribution on x is F and the marginal distribution on y is G.
The error is the probability that x 6= y.
Claim 6.3.2 There is a coupling with error ε between F and G if and only if
dT V (F, G) ≤ ε.
Returning to the problem of clustering the samples from a mixture of two Gaussians,
we have that if dT V (F1 , F2 ) = 1/2 then there is a coupling between F1 and F2
that agrees with probability 1/2. Hence instead of thinking about sampling from a
mixture of two Gaussians in the usual way (choose which component, then choose
a random sample from it) we can alternatively sample as follows:
(a) Choose (x, y) from the best coupling between F1 and F2
(b) If x = y, output x
90
CHAPTER 6. GAUSSIAN MIXTURE MODELS
(c) Else output x with probability w1 , and otherwise output y
This procedure generates a random sample from F , but for half of the samples we
did not need to decide which component generated it at all! Hence even if we knew
the mixture there is no clustering procedure that can correctly classify a polynomial
number of samples into which component generated them! So in the setting where
dT V (F1 , F2 ) is not 1 − o(1), the fundamental approach we have discussed so far does
not work! Nevertheless we will be able to give algorithms to learn the parameters
of F even when dT V (F1 , F2 ) = o(1) and the components almost entirely overlap.
Next we will discuss some of the basic types of goals for learning algorithms:
(a) Improper Density Estimation
Throughout we will assume that F ∈ C where C is some class of distributions (e.g.
mixtures of two Gaussians). Our goal in improper density estimation is to find
any distribution Fb so that dT V (F, Fb) ≤ ε. This is the weakest goal for a learning
algorithm. A popular approach (especially in low dimension) is to construct a kernel
density estimate; suppose we take many samples from F and construct a point-mass
distribution G that represents our samples. Then we can set Fb = G ∗ N (0, σ 2 ), and
if F is smooth enough and we take enough samples, dT V (F, Fb) ≤ ε. However Fb
works without learning anything about the components of F ; it works just because
F is smooth. We remark that such an approach fails badly in high dimensions where
even if F is smooth, we would need to take an exponential number of samples in
order to guarantee that Fb = G ∗ N (0, σ 2 I) is close to F .
(b) Proper Density Estimation
Here, our goal is to find a distribution Fb ∈ C where dT V (F, Fb) ≤ ε. Note that if
C is the set of mixtures of two Gaussians, then a kernel density estimate is not a
valid hypothesis since it will in general be a mixture of many Gaussians (as many
samples as we take). Proper density estimation is in general much harder to do than
improper density estimation. In fact, we will focus on an even stronger goal:
(b) Parameter Learning
Here we require not only that dT V (F, Fb) ≤ ε and that Fb ∈ C, but we want Fb to be
a good estimate for F on a component-by-component basis. For example, our goal
specialized to the case of mixtures of two Gaussians is:
6.4. CLUSTERING-FREE ALGORITHMS
91
Definition 6.3.3 We will say that a mixture Fb = w
b1 Fb1 + w
b2 Fb2 is ε-close (on a
component-by-component basis) to F if there is a permutation π : {1, 2} → {1, 2} so
that for all i ∈ {1, 2}:
bπ(i) , dT V (Fi , Fbπ(i) ) ≤ ε
wi − w
Note that F and Fb must necessarily be close as mixtures too: dT V (F, Fb) ≤ 4ε.
However we can have mixtures F and Fb that are both mixtures of k Gaussians,
are close as distributions but are not close on a component-by-component basis. It
is better to learn F on a component-by-component basis than to do only proper
density estimation, if we can. Note that if Fb is ε-close to F , then even when we
cannot cluster samples we will still be able to approximately compute the posterior
[79] and this is one of the main advantages of parameter learning over some of the
weaker learning goals.
But one should keep in mind that lower bounds for parameter learning do not
imply lower bounds for proper density estimation. We will give optimal algorithms
for parameter learning for mixtures of k Gaussians, which run in polynomial time
for any k = O(1). Moreover there are pairs of mixtures of k Gaussians F and Fb
that are not close on a component-by-component basis, but have dT V (F, Fb) ≤ 2−k
[95]. Hence there is no algorithm for parameter learning that takes poly(n, k, 1/ε)
samples – because we need to take at least 2k samples to distinguish F and Fb. But
in the context of proper density estimation, we do not need to distinguish these two
mixtures.
Open Question 2 Is there a poly(n, k, 1/ε) time algorithm for proper density estimation for mixtures of k Gaussians in n dimensions?
6.4
Clustering-Free Algorithms
Recall, our goal is to learn Fb that is ε-close to F . In fact, the same definition can
be generalized to mixtures of k Gaussians:
Pk
Definition 6.4.1 We will say that a mixture Fb =
bi Fbi is ε-close (on a
i=1 w
component-by-component basis) to F if there is a permutation π : {1, 2, ..., k} →
{1, 2, ..., k} so that for all i ∈ {1, 2, ..., k}:
bπ(i) , dT V (Fi , Fbπ(i) ) ≤ ε
w i − w
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CHAPTER 6. GAUSSIAN MIXTURE MODELS
When can we hope to learn an ε close estimate in poly(n, 1/ε) samples? In
fact, there are two trivial cases where we cannot do this, but these will be the only
things that go wrong:
(a) If wi = 0, we can never learn Fbi that is close to Fi because we never get any
samples from Fi .
In fact, we need a quantitative lower bound on each wi , say wi ≥ ε so that if we
take a reasonable number of samples we will get at least one sample from each
component.
(b) If dT V (Fi , Fj ) = 0 we can never learn wi or wj because Fi and Fj entirely
overlap.
Again, we need a quantitive lower bound on dT V (Fi , Fj ), say dT V (Fi , Fj ) ≥ ε for
each i 6= j so that if we take a reasonable number of samples we will get at least
one sample from the non-overlap region between various pairs of components.
Theorem 6.4.2 [79], [95] If wi ≥ ε for each i and dT V (Fi , Fj ) ≥ ε for each i 6= j,
then there is an efficient algorithm that learns an ε-close estimate Fb to F whose
running time and sample complexity are poly(n, 1/ε, log 1/δ) and succeeds with probability 1 − δ.
Note that the degree of the polynomial depends polynomially on k. Kalai, Moitra
and Valiant [79] gave the first algorithm for learning mixtures of two Gaussians with
no separation conditions. Subsequently Moitra and Valiant [95] gave an algorithm
for mixtures of k Gaussians, again with no separation conditions.
In independent and concurrent work, Belkin and Sinha [23] gave a polynomial
time algorithm for mixtures of k Gaussians too, however there is no explicit bound
given on the running time as a function of k (since their work depends on the basis
theorem, which is provably ineffective). Also, the goal in [79] and [95] is to learn
Fb so that its components are close in total variation distance to those of F , which
is in general a stronger goal than requiring that the parameters be additively close
which is the goal in [23]. The benefit is that the algorithm in [23] works for more
general learning problems in the one-dimensional setting, and we will describe this
algorithm in detail at the end of this chapter.
Throughout this section, we will focus on the k = 2 case since this algorithm
is conceptually much simpler. In fact, we will focus on a weaker learning goal: We
b π(i) kF ≤ ε
will say that Fb is additively ε-close to F if |wi − w
bπ(i) |, kµi − µ
bπ(i) k, kΣi − Σ
for all i. We will further assume that F is normalized appropriately:
6.4. CLUSTERING-FREE ALGORITHMS
93
Definition 6.4.3 A distribution F is in isotropic position if
(a) Ex←F [x] = 0
(b) Ex←F [xxT ] = I
Alternatively, we require that the mean of the distribution is zero and that its
variance in every direction is one. In fact this condition is not quite so strong as it
sounds:
Claim 6.4.4 If Ex←F [xxT ] is full-rank, then there is an affine transformation that
places F in isotropic position
Proof: Let µ = Ex←F [x] and let Ex←F [(x − µ)(x − µ)T ] = M . It is easy to see
that M is positive semi-definite, and in fact is full rank by assumption. Hence we
can write M = BB T where B is invertible (this is often referred to as the Cholesky
decomposition [74]). Then set y = B −1 (x − µ), and it is easy to see that E[y] = 0
and E[yy T ] = B −1 M (B −1 )T = I. Our goal is to learn an additive ε approximation to F , and we will assume that F
has been pre-processed so that it is in isotropic position.
Outline
We can now describe the basic outline of the algorithm, although there will be many
details to fill:
(a) Consider a series of projections down to one dimension
(b) Run a univariate learning algorithm
(c) Set up a system of linear equations on the high-dimensional parameters, and
back solve
Isotropic Projection Lemma
We will need to overcome a number of obstacles to realize this plan, but let us work
through the details of this outline:
Claim 6.4.5 projr [N (µ, Σ)] = N (rT µ, rT Σr)
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CHAPTER 6. GAUSSIAN MIXTURE MODELS
Alternatively, the projection of a high-dimensional Gaussian is a one-dimensional
Gaussian, and its mean and variance are rT µ and rT Σr respectively. This implies
that if we knew the parameters of the projection of a single Gaussian component
onto a (known) direction r, then we could use these parameters to set up a linear
constraint for µ and Σ. If we follow this plan, we would need to consider about n2
projections to get enough linear constraints, since there are Θ(n2 ) variances in Σ
that we need to solve for. Now we will encounter the first problem in the outline.
Let us define some notation:
Definition 6.4.6 dp (N (µ1 , σ12 ), N (µ2 , σ22 )) = |µ1 − µ2 | + |σ12 − σ22 |
We will refer to this as the parameter distance. Ultimately, we will give a univariate
algorithm for learning mixtures of Gaussians and we would like to run it on projr [F ].
Problem 4 But what if dp (projr [F1 ], projr [F2 ]) is exponentially small?
This would be a problem since we would need to run our univariate algorithm with
exponentially fine precision just to see that there are two components and not one!
How can we get around this issue? In fact, this almost surely never happens provided
that F is in isotropic position. For intuition, consider two cases:
(a) Suppose kµ1 − µ2 k ≥ poly(1/n, ε).
If the difference between the means of F1 and F2 is at least any fixed inverse polynomial, then with high probability krT µ1 − rT µ2 k is at least poly(1/n, ε) too. Hence
projr [F1 ] and projr [F2 ] will have different parameters due to a difference in their
means.
(b) Suppose kµ1 − µ2 k ≤ poly(1/n, ε).
The key observation is that if dT V (F1 , F2 ) ≥ ε and their means are almost identical,
then their covariances Σ1 and Σ2 must be noticeably different when projected on a
random direction r. In this case, projr [F1 ] and projr [F2 ] will have different parameters due to a difference in their variances. This is the intuition behind the following
lemma:
Lemma 6.4.7 If F is in isotropic position and wi ≥ ε and dT V (F1 , F2 ) ≥ ε, then
with high probability for a random r
dp (projr [F1 ], projr [F2 ]) ≥ 2ε3 = poly(1/n, ε)
6.4. CLUSTERING-FREE ALGORITHMS
95
Note that this lemma is note true when F is not in isotropic position (e.g. consider
the parallel pancakes example), and moreover when generalizing to mixtures of k > 2
Gaussians this is the key step that fails since even if F is in isotropic position, it
could be that for almost all choices of r the projection onto r results in a mixtures
that is exponentially closet to a mixture of < k Gaussians! (The approach in [95]
is to learn a mixture of < k Gaussians as a proxy for the true mixture, and later
on find a direction that can be used to cluster the mixture into sub mixtures and
recurse).
Pairing Lemma
Next we will encounter the second problem: Suppose we project onto direction r
and s and learn Fbr = 21 Fb1r + 21 Fb2r and Fbs = 21 Fb1s + 21 Fb2s respectively. Then the mean
and variance of Fb1r yield a linear constraint on one of the two high-dimensional
Gaussians, and similarly for Fb1s .
Problem 5 How do we know that they yield constraints on the same high-dimensional
component?
Ultimately we want to set up a system of linear constraints to solve for the
parameters of F1 , but when we project F onto different directions (say, r and s)
we need to pair up the components from these two directions. The key observation
is that as we vary r to s the parameters of the mixture vary continuously. See
Figure ??. Hence when we project onto r, we know from the isotropic projection
lemma that the two components will either have noticeably different means or variances. Suppose their means are different by ε3 ; then if r and s are close (compared
to ε1 ) the parameters of each component in the mixture do not change much and
the component in projr [F ] with larger mean will correspond to the same component
as the one in projs [F ] with larger mean. A similar statement applies when it is the
variances that are at least ε3 apart.
Lemma 6.4.8 If kr − sk ≤ ε2 = poly(1/n, ε3 ) then
(a) If |rT µ1 − rT µ2 | ≥ ε3 then the components in projr [F ] and projs [F ] with the
larger mean correspond to the same high-dimensional component
(b) Else if |rT Σ1 r −rT Σ2 r| ≥ ε3 then the components in projr [F ] and projs [F ] with
the larger variance correspond to the same high-dimensional component
Hence if we choose r randomly and only search over directions s with kr − sk ≤ ε2 ,
we will be able to pair up the components correctly in the different one-dimensional
mixtures.
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CHAPTER 6. GAUSSIAN MIXTURE MODELS
Condition Number Lemma
Now we encounter the final problem in the high-dimensional case: Suppose we choose
r randomly and for s1 , s2 , ...., sp we learn the parameters of the projection of F onto
these directions and pair up the components correctly. We can only hope to learn the
parameters on these projection up to some additive accuracy ε1 (and our univariate
learning algorithm will have running time and sample complexity poly(1/ε1 )).
Problem 6 How do these errors in our univariate estimates translate to errors in
our high dimensional estimates for µ1 , Σ1 , µ2 , Σ2 ?
Recall that the condition number controls this. The final lemma we need in the
high-dimensional case is:
Lemma 6.4.9 The condition number of the linear system to solve for µ1 , Σ1 is
poly(1/ε2 , n) where all pairs of directions are ε2 apart.
Intuitively, as r and s1 , s2 , ...., sp are closer together then the condition number of
the system will be worse (because the linear constraints are closer to redundant),
but the key fact is that the condition number is bounded by a fixed polynomial
in 1/ε2 and n, and hence if we choose ε1 = poly(ε2 , n)ε then our estimates to the
high-dimensional parameters will be within an additive ε. Note that each parameter
ε, ε3 , ε2 , ε1 is a fixed polynomial in the earlier parameters (and 1/n) and hence we
need only run our univariate learning algorithm with inverse polynomial precision
on a polynomial number of mixtures to learn an ε-close estimate Fb!
But we still need to design a univariate algorithm, and next we return to
Pearson’s original problem!
6.5
A Univariate Algorithm
Here we will give a univariate algorithm to learning the parameters of a mixture of
two Gaussians up to additive accuracy ε whose running time and sample complexity
is poly(1/ε). Note that the mixture F = w1 F1 + w2 F2 is in isotropic position (since
the projection of a distribution in isotropic position is itself in isotropic position),
and as before we assume w1 , w2 ≥ ε and dT V (F1 , F2 ) ≥ ε. Our first observation is
that all of the parameters are bounded:
Claim 6.5.1
√
√
(a) µ1 , µ2 ∈ [−1/ ε, 1/ ε]
6.5. A UNIVARIATE ALGORITHM
97
(b) σ12 , σ22 ∈ [0, 1/ε]
This claim is immediate, since if any of the above conditions are violated it would
imply that the mixture has variance strictly larger than one (because w1 , w2 ≥ ε
and the mean of the mixture is zero).
Hence we could try to learn the parameters using a grid search:
Grid Search
Input: samples from F (Θ)
b = (w
Output: parameters Θ
b1 , µ
b1 , σ
b12 , µ
b2 , σ
b22 )
b where the parameters are multiples of εC
For all valid Θ
b using the samples, if it passes output Θ
b
Test Θ
End
b by computing the first six moments of F (Θ) from
For example, we could test out Θ
b if its first six moments are each within an
enough random examples, and output Θ
additive τ of the observed moments. (This is a slight variant on Pearson’s sixth
moment test).
It is easy to see that if we take enough samples and set τ appropriately, then
if we round the true parameters Θ to any valid grid point whose parameters are
b will with high probability pass our test. This
multiples of εC , then the resulting Θ
is called the completeness. The much more challenging part is establishing the
b except for ones close
soundness; after all why is there no other set of parameters Θ
to Θ that pass our test?
Alternatively, we want to prove that any two mixtures F and Fb whose parameters do not match within an additive ε must have one of their first six moments
noticeably different. The main lemma is:
Lemma 6.5.2 For any F and Fb that are not ε-close in parameters, there is an
r ∈ {1, 2, ..., 6} where
b ≥ εO(1)
Mr (Θ) − Mr (Θ)
b are the parameters of F and Fb respectively, and Mr is the rth raw
where Θ and Θ
moment.
fr be the empirical moments. Then
Let M
f b
f
b
f
Mr (Θ) − Mr (Θ) ≤ Mr (Θ) − Mr + Mr − Mr (Θ) ≤ 2τ
{z
} |
{z
}
|
≤τ
≤τ
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CHAPTER 6. GAUSSIAN MIXTURE MODELS
p(x)
F1 (x)
^F (x)
^F (x)
2
1
F2 (x)
^
f(x) = F(x) − F(x)
Figure 6.2: If f (x) has at most six zero crossings, we can find at most degree six
polynomial that agrees with its sign
where the first term is at most τ because the test passes and the second term is
small because we can take enough samples (but still poly(1/τ )) so that the empirical
moments and the true moments are close. Hence we can apply the above lemma in
b then Θ and Θ
b
the contrapositive, and conclude that if the grid search outputs Θ
must be ε-close in parameters, which gives us an efficient univariate algorithm!
So our main goal is to prove that if F and Fb that are not ε-close, then one
of their first six moments is noticeably different. In fact, even the case of ε = 0 is
challenging: If F and Fb are different mixtures of two Gaussians, why is one of their
first six moments necessarily different? Our main goal is to prove this statement,
using the heat equation.
In fact, let us consider the following thought experiment. Let f (x) = F (x) −
b
F (x) be the point-wise difference between the density functions F and Fb. Then,
the heart of the problem is: Can we prove that f (x) crosses the x-axis at most six
times? See Figure 6.2.
Lemma 6.5.3 If f (x) crosses the x-axis at most six times, then one of the first six
moments of F and Fb are different
6.5. A UNIVARIATE ALGORITHM
99
Proof: In fact, we can construct a (non-zero) degree at most six polynomial p(x)
that agrees with the sign of f (x) – i.e. p(x)f (x) ≥ 0 for all x. Then
6
Z
Z X
r
0 < p(x)f (x)dx = pr x f (x)dx
x
x r=1
≤
6
X
b
|pr |Mr (Θ) − Mr (Θ)
r=1
And if the first six moments of F and Fb match exactly, the right hand side is zero
which is a contradiction. So all we need to prove is that F (x) − Fb(x) has at most six zero crossings. Let
us prove a stronger lemma by induction:
P
Lemma 6.5.4 Let f (x) = ki=1 αi N (µi , σi2 , x) be a linear combination of k Gaussians (αi can be negative). Then if f (x) is not identically zero, f (x) has at most
2k − 2 zero crossings.
We will rely on the following tools:
Theorem 6.5.5 Given f (x) : R → R, that is analytic and has n zero crossings, then
for any σ 2 > 0, the function g(x) = f (x) ∗ N (0, σ 2 ) has at most n zero crossings.
This theorem has a physical interpretation. If we think of f (x) as the heat profile
of an infinite one-dimensional rod, then what does the heat profile look like at some
later time? In fact it is precisely g(x) = f (x) ∗ N (0, σ 2 ) for an appropriately chosen
σ 2 . Alternatively, the Gaussian is the Green’s function of the heat equation. And
hence many of our physical intuitions for diffusion have consequences for convolution
– convolving a function by a Gaussian has the effect of smoothing it, and it cannot
create a new local maxima (and relatedly it cannot create new zero crossings).
Finally we recall the elementary fact:
Fact 6.5.6 N (0, σ12 ) ∗ N (0, σ22 ) = N (0, σ12 + σ22 )
Now we are ready to prove the above lemma and conclude that if we knew the
first six moments of a mixture of two Gaussians exactly, then we would know its
parameters exactly too. Let us prove the above lemma by induction, and assume
that for any linear combination of k = 3 Gaussians, the number of zero crossings is
100
CHAPTER 6. GAUSSIAN MIXTURE MODELS
(a) (b) (d) (c) Figure 6.3: (a) linear combination of four Gaussians (b) subtracting σ 2 from each
variance (c) adding back in the delta function (d) convolving by N (0, σ 2 ) to recover
the original linear combination
6.6. A VIEW FROM ALGEBRAIC GEOMETRY
101
at most four. Now consider an arbitrary linear combination of four Gaussians, and
let σ 2 be the smallest variance of any component. See Figure 6.3(a). We can consider
a related mixture where we subtract σ 2 from the variance of each component. See
Figure 6.3(b).
Now if we ignore the delta function, we have a linear combination of three
Gaussians and by induction we know that it has at most four zero crossings. But
how many zero crossings can we add when we add back in the delta function? We
can add at most two, one on the way up and one on the way down (here we are
ignoring some real analysis complications of working with delta functions for ease of
presentation). See Figure 6.3(c). And now we can convolve the function by N (0, σ 2 )
to recover the original linear combination of four Gaussians, but this last step does
not increase the number of zero crossings! See Figure 6.3(d).
This proves that
n
o
b
Mr (Θ) = Mr (Θ) , r = 1, 2, ..., 6
has only two solutions (the true parameters and we can also interchange which is
component is which). In fact, this system of polynomial equations is also stable and
there is an analogue of condition numbers for systems of polynomial equations that
implies a quantitative version of what we have just proved: if F and Fb that are not
ε-close, then one of their first six moments is noticeably different. This gives us our
univariate algorithm.
6.6
A View from Algebraic Geometry
Here we will present an alternative univariate learning algorithm of Belkin and Sinha
[23] that also makes use of the method of moments, but gives a much more general
analysis using tools from algebraic geometry.
Polynomial Families
We will analyze the method of moments for the following class of distributions:
Definition 6.6.1 A class of distributions F (Θ) is called a polynomial family if
∀r, EX∈F (Θ) [X r ] = Mr (Θ)
where Mr (Θ) is a polynomial in Θ = (θ1 , θ2 , ...., θk ).
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CHAPTER 6. GAUSSIAN MIXTURE MODELS
This definition captures a broad class of distributions such as mixtures models whose
components are uniform, exponential, Poisson, Gaussian or gamma functions. We
will need another (tame) condition on the distribution which guarantees that it is
characterized by all of its moments.
P
n
Fact 6.6.2 If the moment generating function (mgf ) of X defined as
E [X n ] tn!
converges in a neighborhood of zero, it uniquely determines the probability distribution, i.e.
b =⇒ F (Θ) = F (Θ).
b
∀r, Mr (Θ) = Mr (Θ)
Our goal is to show that for any polynomial family, a finite number of its moments
suffice. First we introduce the relevant definitions:
Definition 6.6.3 Given a ring R, an ideal I generated by g1 , g2 , · · · , gn ∈ R denoted
by I = hg1 , g2 , · · · , gn i is defined as
(
)
X
I=
ri gi where ri ∈ R .
i
Definition 6.6.4 A Noetherian ring is a ring such that for any sequence of ideals
I1 ⊆ I2 ⊆ I3 ⊆ · · · ,
there is N such that IN = IN +1 = IN +2 = · · · .
Theorem 6.6.5 (Hilbert’s Basis Theorem) If R is a Noetherian ring, then R[X]
is also a Noetherian ring.
It is easy to see that R is a Noetherian ring, and hence we know that R[x] is also
Noetherian. Now we can prove that for any polynomial family, a finite number of
moments suffice to uniquely identify any distribution in the family:
Theorem 6.6.6 Let F (Θ) be a polynomial family. If the moment generating function converges in a neighborhood of zero, there exists N such that
b if and only if Mr (Θ) = Mr (Θ)
b ∀r ∈ 1, 2, · · · , N
F (Θ) = F (Θ)
b = Mr (Θ) − Mr (Θ).
b
Proof: Let Qr (Θ, Θ)
Let I1 = hQ1 i , I2 = hQ1 , Q2 i , · · · .
b We can invoke Hilbert’s basis
This is our ascending chain of ideals in R[Θ, Θ].
6.6. A VIEW FROM ALGEBRAIC GEOMETRY
103
theorem and conclude that R[X] is a Noetherian ring and hence, there is N such
that IN = IN +1 = · · · . So for all N + j, we have
b =
QN +j (Θ, Θ)
N
X
b i (Θ, Θ)
b
pij (Θ, Θ)Q
i=1
b Thus, if Mr (Θ) = Mr (Θ)
b for all r ∈ 1, 2, · · · , N ,
for some polynomial pij ∈ R[Θ, Θ].
b for all r and from Fact 6.6.2 we conclude that F (Θ) = F (Θ).
b
then Mr (Θ) = Mr (Θ)
The other side of the theorem is obvious. The theorem above does not give any finite bound on N , since the basis theorem
does not either. This is because the basis theorem is proved by contradiction, but
more fundamentally it is not possible to give a bound on N that depends only on
the choice of the ring. Consider the following example
Example 1 Consider the Noetherian ring R[x]. Let Ii = xN −i for i = 0, · · · , N .
It is a strictly ascending chain of ideals for i = 0, · · · , N . Therefore, even if the ring
R[x] is fixed, there is no universal bound on N .
Bounds such as those in Theorem 6.6.6 are often referred to as ineffective. Consider
an application of the above result to mixtures of Gaussians: from the above theorem,
we have that any two mixtures F and Fb of k Gaussians are identical if and only if
these mixtures agree on their first N moments. Here N is a function of k, and N is
finite but we cannot write down any explicit bound on N as a function of k using the
above tools. Nevertheless, these tools apply much more broadly than the specialized
ones based on the heat equation that we used to prove that 4k − 2 moments suffice
for mixtures of k Gaussians in the previous section.
Systems of Polynomial Inequalities
In general, we do not have exact access to the moments of a distribution but only
noisy approximations. Our main goal is to prove a quantitive version of the previous
result which shows that any two distributions F and Fb that are close on their first
N moments are close in their parameters too. The key fact is that we can bound
the condition number of systems of polynomial inequalities; there are a number of
ways to do this but we will use quantifier elimination. Recall:
Definition 6.6.7 A set S is semi-algebraic if there exist multivariate polynomials
p1 , ..., pn such that
S = {x1 , ..., xr |pi (x1 , ..., xr ) ≥ 0}
or if S is a finite union or intersection of such sets.
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CHAPTER 6. GAUSSIAN MIXTURE MODELS
Theorem 6.6.8 (Tarski) The projection of a semi-algebraic set is semi-algebraic.
We define the following helper set:
n
o
b : |Mr (Θ) − Mr (Θ)|
b ≤ δ for r = 1, 2, ...N =⇒ kΘ − Θk
b ≤ε .
H(ε, δ) = ∀(Θ, Θ)
Let ε(δ) be the smallest ε as a function of δ:
Theorem 6.6.9 There are fixed constants C1 , C2 , s such that if δ < 1/C1 then
ε(δ) < C2 δ 1/s .
Proof: It is easy to see that we can define H(ε, δ) as the projection of a semialgebraic set, and hence using Tarski’s theorem we conclude that H(ε, δ) is also
semi-algebraic. The crucial observation is that because H(ε, δ) is semi-algebraic,
the smallest that we can choose ε to be as a function of δ is itself a polynomial
function of δ. There are some caveats here, because we need to prove that for a
fixed δ we can choose ε to be strictly greater than zero and moreover the polynomial
relationship between ε and δ only holds if δ is sufficiently small. However these
technical issues can be resolved without much more work, see [23] and the main
result is the following. b ≤
Corollary 6.6.10 If |Mr (Θ) − Mr (Θ)|
ε
C2
s
b ≤ ε.
then |Θ − Θ|
Hence there is a polynomial time algorithm to learn the parameters of any univariate polynomial family (whose mgf converges in a neighborhood of zero) within
an additive accuracy of ε whose running time and sample complexity is poly(1/ε);
we can take enough samples to estimate the first N moments within εs and search
over a grid of the parameters, and any set of parameters that matches each of the
moments is necessarily close in parameter distance to the true parameters.
Chapter 7
Matrix Completion
Here we will give algorithms for the matrix completion problem, where we observe
uniformly random entries of a low-rank, incoherent matrix M and we would like
design efficient algorithms that exactly recover M .
7.1
Background
The usual motivation for studying the matrix completion problem comes from recommendation systems. To be concrete, consider the Netflix problem where we are
given ratings Mi,j that represent how user i rated movie j. We would like to use
these ratings to make good recommendations to users, and a standard approach is
to try to use our knowledge of some of the entries of M to fill in the rest of M .
Let us be more precise: There is an unknown matrix M ∈ Rn×m whose rows
represent users and whose columns represent movies in the example above. For each
(i, j) ∈ Ω ⊆ [n] × [m] we are given the value Mi,j . Our goal is to recover M exactly.
Ideally, we would like to find the minimum rank matrix X that agrees with M on
the observed entries {Mi,j }(i,j)∈Ω however this problem is N P -hard. There are some
now standard assumptions under which we will be able to give efficient algorithms
for recovering M exactly:
(a) Ω is uniformly random
(b) The singular vectors of M are uncorrelated with the standard basis (such a
matrix is called incoherent and we define this later)
In fact, we will see that there are efficient algorithms for recovering M exactly if
m ≈ mr log m where m ≥ n and rank(M ) ≤ r. This is similar to compressed
105
106
CHAPTER 7. MATRIX COMPLETION
sensing, where we were able to recover a k-sparse signal x from O(k log n/k) linear
measurements, which is much smaller than the dimension of x. Here too we can
recover a low-rank matrix M from a number of observations that is much smaller
than the dimension of M .
Let us examine the assumptions above. The assumption that should give us
pause is that Ω is uniformly random. This is somewhat unnatural since it would
be more believable if the probability we observe Mi,j depends on the value itself.
Alternatively, a user should be more likely to rate a movie if he actually liked it.
In order to understand the second assumption, suppose Ω is indeed uniformly
random. Consider
Ir 0
ΠT
M =Π
0 0
where Π is a uniformly random permutation matrix. M is low-rank, but unless we
observe all of the ones along the diagonal, we will not be able to recover M uniquely.
Indeed, the singular vectors of M contain some of the standard basis vectors; but
if we were to assume that the singular vectors of M are incoherent with respect to
the standard basis, we could avoid the above problem.
Definition 7.1.1 The coherence µ of a subspace U ⊆ Rn of dimension dim(u) = r
is
n
max kPU ei k2 ,
r i
where PU denotes the orthogonal projection onto U , and ei is the standard basis
element.
e
It is easy to see that if we choose U uniformly at random, then µ(U ) = O(1).
Also
we have that 1 ≤ µ(U ) ≤ n/r and the upper bound is attained if U contains any ei .
We can now see that if we set U to be the top singular vectors of the above example,
then U has high coherence. We will need the following conditions on M :
(a) Let M = U ΣV T , then µ(U ), µ(V ) ≤ µ0 .
(b) kU V T k∞ ≤
entry.
√
µ1 r
,
n
where || · ||∞ denotes the maximum absolute value of any
The main result of this chapter is:
Theorem 7.1.2 Suppose Ω is chosen uniformly at random. Then there is a polynomial time algorithm to recover M exactly that succeeds with high probability if
m ≥ max(µ21 , µ0 )r(n + m) log2 (n + m)
7.2. NUCLEAR NORM
107
The algorithm in the theorem above is based on a convex relaxation for the rank
of a matrix called the nuclear norm. We will introduce this in the next section,
and establish some of its properties but one can think of it as an analogue to the
`1 minimization approach that we used in compressed sensing. This approach was
first introduced in Fazel’s thesis [58], and Recht, Fazel and Parrilo [104] proved that
this approach exactly recovers M in the setting of matrix sensing, which is related
to the problem we consider here.
In a landmark paper, Candes and Recht [33] proved that the relaxation based
on nuclear norm also succeeds for matrix completion and introduced the assumptions
above in order to prove that their algorithm works. There has since been a long line
of work improving the requirements on m, and the theorem above and our exposition
will follow a recent paper of Recht [103] that greatly simplifies the analysis by making
use of matrix analogues of the Bernstein bound and using these in a procedure now
called quantum golfing that was first introduced by Gross [67].
e
Remark 7.1.3 We will restrict to M ∈ Rn×n and assume µ0 , µ1 = O(1)
in our
analysis, which will reduce the number of parameters we need to keep track of. Also
let m = n.
7.2
Nuclear Norm
Here we introduce the nuclear norm, which will be the basis for our algorithms for
matrix completion. We will follow a parallel outline to that of compressed sensing.
In particular, a natural starting point is the optimization problem:
(P 0)
min rank(X) s.t. Xi,j = Mi,j for all (i, j) ∈ Ω
This optimization problem is N P -hard. If σ(X) is the vector of singular values of
X then we can think of the rank of X equivalently as the sparsity of σ(X). Recall,
in compressed sensing we faced a similar obstacle: finding the sparsest solution
to a system of linear equations is also N P -hard, but we instead considered the
`1 relaxation and proved that under various conditions this optimization problem
recovers the sparsest solution. Similarly it is natural to consider the `1 -norm of σ(X)
which is called the nuclear norm:
Definition 7.2.1 The nuclear norm of X denoted by kXk∗ is kσ(X)k1 .
We will instead solve the convex program:
(P 1)
min kXk∗ s.t. Xi,j = Mi,j for all (i, j) ∈ Ω
108
CHAPTER 7. MATRIX COMPLETION
and our goal is to prove conditions under which the solution to (P 1) is exactly M .
Note that this is a convex program because kXk∗ is a norm, and there are a variety
of efficient algorithms to solve the above program.
In fact, for our purposes a crucial notion is that of a dual norm. We will not
need this concept in full-generality, so we state it for the specific case of the nuclear
norm. This concept gives us a method to lower bound the nuclear norm of a matrix:
Definition 7.2.2 Let hX, Bi =
product.
P
i,j
Xi,j Bi,j = trace(X T B) denote the matrix inner-
Lemma 7.2.3 kXk∗ = maxkBk≤1 hX, Bi.
To get a feel for this, consider the special case where we restrict X and B to be
diagonal. Moreover let X = diag(x) and B = diag(b). Then kXk∗ = kxk1 and
the constraint kBk ≤ 1 (the spectral norm of B is at most one) is equivalent to
kbk∞ ≤ 1. So we can recover a more familiar characterization of vector norms in
the special case of diagonal matrices:
kxk1 = max bT x
kbk∞ ≤1
Proof: We will only prove one direction of the above lemma. What B should we
use to certify the nuclear norm of X. Let X = UX ΣX VXT , then we will choose
B = UX VXT . Then
hX, Bi = trace(B T X) = trace(VX UXT UX ΣX VXT ) = trace(VX ΣX VXT ) = trace(ΣX ) = kXk∗
where we have used the basic fact that trace(ABC) = trace(BCA). Hence this
proves kXk∗ ≤ maxkBk≤1 hX, Bi, and the other direction is not much more difficult
(see e.g. [74]). How can we show that the solution to (P 1) is M ? Our basic approach will
be a proof by contradiction. Suppose not, then the solution is M + Z for some Z
that is supported in Ω. Our goal will be to construct a matrix B of spectral norm
at most one for which
kM + Zk∗ ≥ hM + Z, Bi > kM k∗
Hence M + Z would not be the optimal solution to (P 1). This strategy is similar to
the one in compressed sensing, where we hypothesized some other solution w that
differs from x by a vector y in the kernel of the sensing matrix A. We used geometric
7.2. NUCLEAR NORM
109
properties of ker(A) to prove that w has strictly larger `1 norm than x. However the
proof here will be more involved since our strategy is to construct B above based
on Z (rather than relying on some geometry property of A that holds regardless of
what y is).
Let us introduce some basic projection operators that will be crucial in our
proof. Recall, M = U ΣV T , let u1 , . . . , ur be columns of U and let v1 , . . . , vr be
columns of V . Choose ur+1 , . . . , un so that u1 , . . . , un form an orthonormal basis
for all of Rn – i.e. ur+1 , . . . , un is an arbitrary orthonormal basis of U ⊥ . Similarly
choose vr+1 , . . . , vn so that v1 , . . . , vn form an orthonormal basis for all of Rn . We
will be interested in the following linear spaces over matrices:
Definition 7.2.4 T = span{ui vjT | 1 ≤ i ≤ r or 1 ≤ j ≤ r or both}.
Then T ⊥ = span{ui vjT s.t. r + 1 ≤ i, j ≤ n}.. We have dim(T ) = r2 + 2(n − r)r and
dim(T ⊥ ) = (n − r)2 . Moreover we can define the linear operators that project into
T and T ⊥ respectively:
PT ⊥ [Z] =
n
X
hZ, ui vjT i · Ui vjT = PU ⊥ ZPV ⊥ .
i,j=r+1
And similarly
PT [Z] =
X
hZ, ui vjT i · ui vjT = PU Z + ZPV − PU ZPV .
(i,j)∈[n]×[n]−[r+1,n]×[r+1,n]
We are now ready to describe the outline of the proof of Theorem 7.1.2. The
proof will be based on:
(a) We will assume that a certain helper matrix Y exists, and show that this is
enough to imply kM + Zk∗ > kM k∗ for any Z supported in Ω
(b) We will construct such a Y using quantum golfing [67].
Part (a)
Here we will state the conditions we need on the helper matrix Y and prove that if
such a Y exists, then M is the solution to (P 1). We require that Y is supported in
Ω and
(a) kPT (Y ) − U V T kF ≤
p
r/8n
110
CHAPTER 7. MATRIX COMPLETION
(b) kPT ⊥ (Y )k ≤ 1/2.
We want to prove that for any Z supported in Ω, kM + Zk∗ > kM k∗ . Recall,
we want to find a matrix B of spectral norm at most one so that hM +Z, Bi > kM k∗ .
Let U⊥ and V⊥ be singular vectors of PT ⊥ [Z]. Then consider
T V
= U V T + U⊥ V⊥T .
B = U U⊥ ·
V⊥T
Claim 7.2.5 kBk ≤ 1
Proof: By construction U T U⊥ = 0 and V T V⊥ = 0 and hence the above expression
for B is its singular value decomposition, and the claim now follows. Hence we can plug in our choice for B and simplify:
kM + Zk∗ ≥ hM + Z, Bi
= hM + Z, U V T + U⊥ V⊥T i
= hM, U V T i +hZ, U V T + U⊥ V⊥T i
| {z }
kM k∗
where in the last line we used the fact that M is orthogonal to U⊥ V⊥T . Now using
the fact that Y and Z have disjoint supports we can conclude:
kM + Zk∗ ≥ kM k∗ + hZ, U V T + U⊥ V⊥T − Y i
Therefore in order to prove the main result in this section it suffices to prove that
hZ, U V T + U⊥ V⊥T − Y i > 0. We can expand this quantity in terms of its projection
onto T and T ⊥ and simplify as follows:
kM + Zk∗ − kM k∗ ≥ hPT (Z), PT (U V T + U⊥ V⊥T − Y )i + hPT ⊥ (Z), PT ⊥ (U V T + U⊥ V⊥T − Y )i
≥ hPT (Z), U V T − PT (Y )i + hPT ⊥ (Z), U⊥ V⊥T − PT ⊥ (Y )i
≥ hPT (Z), U V T − PT (Y )i + kPT ⊥ (Z)k∗ − hPT ⊥ (Z), PT ⊥ (Y )i
where in the last line we used the fact that U⊥ and V⊥ are the singular vectors of
PT ⊥ [Z] and hence hU⊥ V⊥T , PT ⊥ [Z]i = kPT ⊥ [Z]k∗ .
Now we can invoke the properties of Y that we have assumed in this section,
to prove a lower bound
the right hand side. By property (a) of Y , we have that
p on
r
T
kPT (Y ) − U V kF ≤ 2n . Therefore, we know that the first term hPT (Z), U V T −
pr
PT (Y )i ≥ − 8n
kPT (Z)kF . By property (b) of Y , we know the operator norm
7.3. QUANTUM GOLFING
111
of PT⊥ (Y ) is at most 1/2. Therefore the third term hPT ⊥ (Z), PT ⊥ (Y )i is at most
1
kPT ⊥ (Z)k∗ . Hence
2
r
?
r
1
kM + Zk∗ − kM k∗ ≥ −
kPT (Z)kF + kPT ⊥ (Z)k∗ > 0
8n
2
We will show that with high probability over the choice of Ω that the inequality
does indeed hold. We defer the proof of this last fact, since it and the construction
of the helper matrix Y will both make use of the matrix Bernstein inequality which
we present in the next section.
7.3
Quantum Golfing
What remains is to construct a helper matrix Y and prove that with
probability
phigh
r
kP
over Ω, for any matrix Z supported in Ω that kPT ⊥ (Z)k∗ >
T (Z)kF to
2n
complete the proof we started in the previous section. We will make use of an
approach introduced by Gross [67] and we will follow the proof of Recht in [103]
where the strategy is to construct Y iteratively. In each phase, we will invoke
concentration results for matrix valued random variables to prove that the error
part of Y decreases geometrically and we make rapid progress in constructing a
good helper matrix.
First we will introduce the key concentration result that we will apply in several
settings. The following matrix valued Bernstein inequality first appeared in the work
of Ahlswede and Winter related to quantum information theory [6].
Theorem 7.3.1 (Non-commutative Bernstein Inequality) Let X1 . . . Xl be independent mean 0 matrices of size d × d. Let ρ2k = max{k E[Xk XkT ]k, k E[XkT Xk ]k}
and suppose kXk k ≤ M almost surely. Then for τ > 0,
" l
#
X −τ 2 /2
Xk > τ ≤ 2d exp P 2
Pr k ρk + M τ /3
k=1
If d = 1 this is the standard Bernstein inequality. If d > 1 and the matrices Xk are
diagonal then this inequality can be obtained from the union bound and the standard
Bernstein inequality again. However to build intuition, consider the following toy
problem. Let uk be a random unit vector in Rd and let Xk = P
uk uTk . Then it is easy
2
to see that ρk = 1/d. How many trials do we need so that k Xk is close to the
identity (after scaling)? We should expect to need Θ(d log d) trials; this is even true
if uk is drawn uniformly at random from the standard basis vectors {e1 . . . ed } due to
112
CHAPTER 7. MATRIX COMPLETION
the coupon collector problem. Indeed, the above bound corroborates our intuition
that Θ(d log d) is necessary and sufficient.
Now we will apply the above inequality to build up the tools we will need to
finish the proof.
Definition 7.3.2 Let RΩ be the operator that zeros out all the entries of a matrix
except those in Ω.
Lemma 7.3.3 If Ω is chosen uniformly at random and m ≥ nr log n then with high
probability
m n2 1
P
R
P
−
P
T Ω T
<
T
m
n2
2
Remark 7.3.4 Here we are interested in bounding the operator norm of a linear
operator on matrices. Let T be such an operator, then kT k is defined as
max kT (Z)kF
kZkF ≤1
We will explain how this bound fits into the framework of the matrix Bernstein
inequality, but for a full proof see [103]. Note that E[PT RΩ PT ] = PT E[RΩ ]PT =
m
P and so we just need to show that PT RΩ PT does not deviate too far from its
n2 T
expectation. Let e1 , e2 , . . . , ed be the standard basis vectors. Then we can expand:
X
hPT (Z), ea eTb iea eTb
PT (Z) =
a,b
X
hPT (Z), ea eTb iea eTb
=
a,b
X
=
hZ, PT (ea eTb )iea eTb
a,b
Hence RΩ PT (Z) =
P
(a,b)∈Ω
Z, PT (ea eTb ) ea eTb and finally we conclude that
PT RΩ PT (Z) =
X Z, PT (ea eTb ) PT (ea eTb )
(a,b)∈Ω
We
can think
of PT RTΩ PT as the sum of random operators of the form τa,b : Z →
T
Z, PT (ea eb ) PT (ea eb ), and the lemma
follows by applying the matrix Bernstein
P
inequality to the random operator (a,b)∈Ω τa,b .
We can now complete the deferred proof of part (a):
7.3. QUANTUM GOLFING
113
Lemma 7.3.5 If Ω is chosen uniformly at random and m ≥ nr log n then with high
probability for any Z supported in Ω we have
r
r
kPT ⊥ (Z)k∗ >
kPT (Z)kF
2n
Proof: Using Lemma 7.3.3 and the definition of the operator norm (see the remark)
we have
D
E
m
m
Z, PT RΩ PT Z − 2 PT Z ≥ − 2 kZk2F
n
2n
Furthermore we can upper bound the left hand side as:
2
hZ, PT RΩ PT Zi = hZ, PT RΩ
PT Zi = kRΩ (Z − PT ⊥ (Z))k2F
= kRΩ (PT ⊥ (Z))k2F ≤ kPT ⊥ (Z)k2F
where in the last line we used that Z is supported in Ω and so RΩ (Z) = 0. Hence
we have that
m
m
kPT ⊥ (Z)k2F ≥ 2 kPT (Z)k2F − 2 kZk2F
n
2n
We can use the fact that kZk2F = kPT ⊥ (Z)k2F +kPT (Z)k2F and conclude kPT ⊥ (Z)k2F ≥
m
kPT (Z)k2F . We can now complete the proof of the lemma
4n2
m
kPT ⊥ (Z)k2∗ ≥ kPT ⊥ (Z)k2F ≥ 2 kPT (Z)k2F
4n
r
>
kPT (Z)k2F
2n
All that remains is to prove that the helper matrix Y that we made use of
actually does exists (with high probability).
Recall that we require that Y is supp
T
ported in Ω and kPT (Y ) − U V kF ≤ r/8n and kPT ⊥ (Y )k ≤ 1/2. The basic idea
is to break up Ω into disjoint sets Ω1 , Ω2 , . . . Ωp , where p = log n and use each set
of observations to make progress on the remained PT (Y ) − U V T . More precisely,
initialize Y0 = 0 in which case the remainder is W0 = U V T . Then set
Yi+1 = Yi +
n2
RΩ (Wi )
m i+1
2
and update Wi+1 = U V T − PT (Yi+1 ). It is easy to see that E[ nm RΩi+1 ] = I. Intuitively this means that at each step Yi+1 − Yi is an unbiased estimator for Wi and so
we should expect the remainder to decrease quickly (here we will rely on the concentration bounds we derived from the non-commutative Bernstein inequality). Now
114
CHAPTER 7. MATRIX COMPLETION
we can explain the nomenclature quantum golfing; at each step, we hit our golf ball
in the direction of the hole but here our target is to approximate the matrix U V T
which for various reasons is the type of question that arises in quantum mechanics.
P
It is easy to see that Y = i Yi is supported in Ω and that PT (Wi ) = Wi for
all i. Hence we can compute
n2
n2
T
kPT (Yi ) − U V kF = Wi−1 − PT RΩi Wi−1 = PT Wi−1 − PT RΩi PT Wi−1 m
m
F
F
1
n2 m
≤
PT RΩ PT − 2 PT ≤ kWi−1 kF
m
n
2
where the last inequality follows from Lemma 7.3.3. Therefore the Frobenius norm
of the remainder decreases geometrically and it is easy to guarantee that Y satisfies
condition (a).
The more technically involved part is showing that Y also satisfies condition
(b). However the intuition is that kPT ⊥ (Y1 )k is itself not too large, and since the
norm of the remainder Wi decreases geometrically we should expect that kPT ⊥ (Yi )k
does too and so most of the contribution to
X
kPT ⊥ (Y )k ≤
kPT ⊥ (Yi )k
i
comes from the first term. For the full details see [103]. This completes the proof
that computing the solution to the convex program indeed finds M exactly, provided
that M is incoherent and |Ω| ≥ max(µ21 , µ0 )r(n + m) log2 (n + m).
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