MATH0042 Exam 2019
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MATH0042
You are given the following formulae which you may use without proof in your answers, unless otherwise stated.
Geometric series:
A=》
Maclaurin series expansion of the exponential function:
exp(x) = , Vx e 皿. (2)
Maclaurin series expansion of the sine function:
sin(x) = x_A辛4 ,
Maclaurin series expansion of the cosine function:
cos(x) = x_A ,
Recurrence relation for Legendre polynomials:
P》(x) = 1, P4 (x) = x,
PA (x) = [x (2n - 1) PA一4 (x) - (n - 1) PA一_ (x)] , for n 2 2.
(3)
(4)
(5a)
(5b)
Orthogonality of Legendre polynomials:
,0
(PA , P衿)= ( 2
| 2n + 1
if n m,
if n = m.
(6)
1. Consider the second order ordinary differential equation
(x_ + x) y′′ (x) + 3x y′ (x) + y(x) = 0, -1 < x < 1.
(a) Determine and classify ordinary and singular points.
(b) Seeking a solution of the form y(x) = x岁 L aA xA , a》 0, show that the roots
of the indicial equation r4 and r_ , r4 < r_ , differ by an integer. Write down the recurrence relation for the coefficients.
(c) Determine the series solution corresponding to r_ .
(d) Show that the solution found in part (c) can be written as
α x
y(x) =
where α is a real constant.
2. Consider the second order ordinary differential equation
x y′′ (x) - y′ (x) + 4x³ y(x) = 0. (7)
(a) Show that the substitution t = x_ turns (7) into an ODE with constant coeffi-
cients.
(b) Using elementary methods, find the general solution to the ODE from part (a).
Express the result in terms of x.
(c) Seeking a solution of (7) in the form y(x) = L aA xA辛岁 , a》 0, find the roots r4 and r_ , r4 < r_ , of the indicial equation. You can assume that x = 0 is a regular singular point for (7).
(d) Show that for r = r_ , the largest root of the indicial equation, the coefficients satisfy
a4 = a_ = a³ = 0,
aA = - aA一4 for n 2 4.
Hence conclude, providing justification, that the corresponding solution reads
&
y(x) =↓ a4A x4A辛_ = a》 sin(x_ ).
A=》
3. Consider Legendre’s equation
(1 - x_ ) y′′ (x) - 2x y′ (x) + n(n + 1) y(x) = 0, -1 < x < 1.
(a) Define what the Legendre polynomial PA (x) is, in relation to the above equation. (b) Show that (PA , P衿)= 0 for n m.
(c) Let f : [-1, 1] - 皿 be a piecewise continuous function with finite jumps and consider its Legendre series expansion f (x) =L aA PA (x). Derive a formula for aA in terms of f and PA . You may use without proof formula (6).
(d) Compute P_ (x) from the recurrence relation (5a)-(5b) and check that it is an even function.
4. When looking for a radial solution R(ρ) to the Schr¨odinger’s equation for the hy- drogen atom, one needs to study the ordinary differential equation
R′′ (ρ) + R′ (ρ) - R(ρ) + ╱ - 、R(ρ) = 0, (8)
where e is an integer number and γ a physical constant. The solution R(ρ) is required to satisfy the normalisation condition
|》& IR(ρ)I_ ρ_ dρ = 1. (9)
(a) Show that R(ρ) s e一/λ_ for ρ - +o.
(b) Writing R(ρ) = e一/λ_ G(ρ), show that G satisfies the differential equation
G′′ (ρ) + ╱ - 1、G′ (ρ) + ┌ - ┐ G(ρ) = 0.
(c) Show that G(ρ) s ρ卜 for ρ - 0.
(d) When γ = 1 and e = 0, a solution to (8) is given by R(ρ) = C e一/λ_. Determine the positive constant C such that the normalisation condition (9) is satisfied.
5. (a) Let M be a 2 ×2 complex matrix and let p杜 (λ) be its characteristic polynomial.
Show that p杜 (λ) = λ_ - Tr(M) λ + det(M).
(b) State the Cayley-Hamilton theorem.
(c) Let
B = ╱ 、
)-9 0 6 | .
Compute B_》49 .
(d) Using the Cayley-Hamilton theorem, find the inverse of A = 、.
6. (a) Check that the matrix
H = 、
is Hermitian. Is it normal?
(b) Compute the eigenvalues λ 4 , λ_ of H and the corresponding eigenvectors v4 , v_ .
Verify that
(i) the eigenvalues λ 4 and λ_ satisfy the theorem about the spectrum of Her- mitian matrices;
(ii) the eigenvectors v4 and v_ are orthogonal.
(c) Write down a unitary matrix U such that U一4 HU is diagonal.
(d) Let N be a square matrix such that N³ = 0. Show that all eigenvalues of N are zero.
7. (a) Define the following groups: GL(n, 皿), SL(n, 皿), O(n), SO(n).
(b) Show that
G = ,╱0(1) 1(z)、 │ z e c、
equipped with standard matrix multiplication is a group. Is it Abelian?
(c) Write down the group table for C4 , the cyclic group of order 4.
(d) Write down the matrix representation of the transformation of 皿³ obtained by performing a translation by the vector
v = ╱ 0(1) 、
)-2|
followed by a counter-clockwise rotation of an angle π/6 about the z-axis.
2022-05-19