# N21a.

```Mathematical Methods I
Dr M B Wingate
Natural Sciences Tripos, Part IB
Michaelmas Term 2012
Example Sheet 0
Copyright © 2012 University of Cambridge. Not to be quoted or reproduced without permission.
This is a revision sheet. If you did NST Mathematics A or B last year you should be able
to do the questions already (let me know if I have made an incorrect assumption here,
especially if you did Course A). If you did the Mathematical Tripos last year you will have
to read up on Fourier Series for question 4. Some of the material will be touched on in
the first couple of lectures, so you might prefer to wait until then.
1. Let f (x) be a function of one variable. Working to O((δx)3 ), write down the Taylor
expansion of f (x + δx). Let g(x, y, z) be a function of three variables. Working to
O(δx, δy, δz), write down the Taylor expansion of g(x + δx, y + δy, z + δz). Express
the latter using vector notation.
2. (a) Let h be a function of one variable. Working from first principles, differentiate
with respect to x the function
Z x
h(y) dy ,
I(x) =
a
where a is a constant.
(b) Let f (x, y) be a function of two variables. Working from first principles, differentiate with respect to x the function
Z x
f (x, y) dy .
J(x) =
a
3. Find functions y(x) which satisfy
x2
d2 y
dy
+ 3x
− 15y = 0 .
2
dx
dx
4. (a) Let f (y) be an odd periodic function of y with period 2, i.e. f (−y) = −f (y)
and f (y + 2) = f (y). Given that f (y) = 12 y(y − 1) for 0 6 y 6 1, sketch the
function f (y) for −2 6 y 6 2, and find the Fourier (sine) series for f .
(b) Let g(x) be an even periodic function of x with period 4L. Given that g(x) =
(x − L)/L for 0 6 x 6 2L, sketch g(x) for −4L 6 x 6 4L and find the Fourier
series for g.
(c) Let h(x) be a periodic function of x with period 2 ln 2, where
½
1
for − ln 2 < x 6 0
h(x) =
x
e − 1 for 0 < x 6 ln 2 .
Find the Fourier series for h.
1
5. Show the following:
(a) For vector components ai (i = 1, 2, 3),
3
X
aj δij = ai ;
j=1
where δij is the Kronecker delta,
Copyright © 2012 University of Cambridge. Not to be quoted or reproduced without permission.
(b) For independent variables qi (i = 1, 2, 3),
∂qi
= δij .
∂qj
6. Show that, if ei · ej = δij (i, j = 1, 2, 3) and e1 × e2 = e3 , then
e2 × e3 = e1
and e3 × e1 = e2 .
7. Using a coordinate system of your choice, write down an example function possessing
each of the following kinds of symmetry:
(a) A function of 2 variables which has circular symmetry
(b) A function of 3 variables which is spherically symmetric
(c) A function of 3 variables which has axial symmetry
8. Evaluate ∇ · r and ∇ × r, where r is the position vector.
9. Show that, if v = ω × r, where ω is a constant vector and r is the position vector,
then ∇ × v = 2ω. (Feel free to use suffix notation.) Comment.
10. Let ψ(r) and χ(r) be scalar fields, and u(r) and v(r) vector fields. Show the
following (feel free to use suffix notation):
(a)
∇ × ψv = ∇ψ × v + ψ∇ × v ,
(b)
∇·u×v = v·∇×u−u·∇×v,
and hence that
∇ · (∇ψ × ∇χ) = 0 .
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