Part II, 2016, Paper 2

Algebraic Geometry

• Paper 2, Section II, H

  Algebraic Geometry | Part II, 2016

  In this question we work over an algebraically closed field of characteristic zero. Let $X^{o}=Z\left(x^{6}+x y^{5}+y^{6}-1\right) \subset \mathbb{A}^{2}$ and let $X \subset \mathbb{P}^{2}$ be the closure of $X^{o}$ in $\mathbb{P}^{2} .$

  (a) Show that $X$ is a non-singular curve.

  (b) Show that $\omega=d x /\left(5 x y^{4}+6 y^{5}\right)$ is a regular differential on $X$.

  (c) Compute the divisor of $\omega$. What is the genus of $X$ ?

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Algebraic Topology

• Paper 2, Section II, G

  Algebraic Topology | Part II, 2016

  (a) Let $K, L$ be simplicial complexes, and $f:|K| \rightarrow|L|$ a continuous map. What does it mean to say that $g: K \rightarrow L$ is a simplicial approximation to $f ?$

  (b) Define the barycentric subdivision of a simplicial complex $K$, and state the Simplicial Approximation Theorem.

  (c) Show that if $g$ is a simplicial approximation to $f$ then $f \simeq|g|$.

  (d) Show that the natural inclusion $\left|K^{(1)}\right| \rightarrow|K|$ induces a surjective map on fundamental groups.

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Applications of Quantum Mechanics

• Paper 2, Section II, A

  Applications of Quantum Mechanics | Part II, 2016

  A particle of mass $m$ moves in three dimensions subject to a potential $V(\mathbf{r})$ localised near the origin. The wavefunction for a scattering process with incident particle of wavevector $\mathbf{k}$ is denoted $\psi(\mathbf{k}, \mathbf{r})$. With reference to the asymptotic form of $\psi$, define the scattering amplitude $f\left(\mathbf{k}, \mathbf{k}^{\prime}\right)$, where $\mathbf{k}^{\prime}$ is the wavevector of the outgoing particle with $\left|\mathbf{k}^{\prime}\right|=|\mathbf{k}|=k$.

  By recasting the Schrödinger equation for $\psi(\mathbf{k}, \mathbf{r})$ as an integral equation, show that

  $$f\left(\mathbf{k}, \mathbf{k}^{\prime}\right)=-\frac{m}{2 \pi \hbar^{2}} \int d^{3} \mathbf{r}^{\prime} \exp \left(-i \mathbf{k}^{\prime} \cdot \mathbf{r}^{\prime}\right) V\left(\mathbf{r}^{\prime}\right) \psi\left(\mathbf{k}, \mathbf{r}^{\prime}\right)$$

  [You may assume that

  $$\mathcal{G}(k ; \mathbf{r})=-\frac{1}{4 \pi|\mathbf{r}|} \exp (i k|\mathbf{r}|)$$

  is the Green's function for $\nabla^{2}+k^{2}$ which obeys the appropriate boundary conditions for a scattering solution.]

  Now suppose $V(\mathbf{r})=\lambda U(\mathbf{r})$, where $\lambda \ll 1$ is a dimensionless constant. Determine the first two non-zero terms in the expansion of $f\left(\mathbf{k}, \mathbf{k}^{\prime}\right)$ in powers of $\lambda$, giving each term explicitly as an integral over one or more position variables $\mathbf{r}, \mathbf{r}^{\prime}, \ldots$

  Evaluate the contribution to $f\left(\mathbf{k}, \mathbf{k}^{\prime}\right)$ of order $\lambda$ in the case $U(\mathbf{r})=\delta(|\mathbf{r}|-a)$, expressing the answer as a function of $a, k$ and the scattering angle $\theta$ (defined so that $\left.\mathbf{k} \cdot \mathbf{k}^{\prime}=k^{2} \cos \theta\right)$.

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Applied Probability

• Paper 2, Section II, J

  Applied Probability | Part II, 2016

  (a) Define an $M / M / \infty$ queue and write without proof its stationary distribution. State Burke's theorem for an $M / M / \infty$ queue.

  (b) Let $X$ be an $M / M / \infty$ queue with arrival rate $\lambda$ and service rate $\mu$ started from the stationary distribution. For each $t$, denote by $A_{1}(t)$ the last time before $t$ that a customer departed the queue and $A_{2}(t)$ the first time after $t$ that a customer departed the queue. If there is no arrival before time $t$, then we set $A_{1}(t)=0$. What is the limit as $t \rightarrow \infty$ of $\mathbb{E}\left[A_{2}(t)-A_{1}(t)\right]$ ? Explain.

  (c) Consider a system of $N$ queues serving a finite number $K$ of customers in the following way: at station $1 \leqslant i \leqslant N$, customers are served immediately and the service times are independent exponentially distributed with parameter $\mu_{i}$; after service, each customer goes to station $j$ with probability $p_{i j}>0$. We assume here that the system is closed, i.e., $\sum_{j} p_{i j}=1$ for all $1 \leqslant i \leqslant N$.

  Let $S=\left\{\left(n_{1}, \ldots, n_{N}\right): n_{i} \in \mathbb{N}, \sum_{i=1}^{N} n_{i}=K\right\}$ be the state space of the Markov chain. Write down its $Q$-matrix. Also write down the $Q$-matrix $R$ corresponding to the position in the network of one customer (that is, when $K=1$ ). Show that there is a unique distribution $\left(\lambda_{i}\right)_{1 \leqslant i \leqslant N}$ such that $\lambda R=0$. Show that

  $$\pi(n)=C_{N} \prod_{i=1}^{N} \frac{\lambda_{i}^{n_{i}}}{n_{i} !}, \quad n=\left(n_{1}, \ldots, n_{N}\right) \in S$$

  defines an invariant measure for the chain. Are the queue lengths independent at equilibrium?

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Asymptotic Methods

• Paper 2, Section II, C

  Asymptotic Methods | Part II, 2016

  What is meant by the asymptotic relation

  $$f(z) \sim g(z) \quad \text { as } \quad z \rightarrow z_{0}, \operatorname{Arg}\left(z-z_{0}\right) \in\left(\theta_{0}, \theta_{1}\right) ?$$

  Show that

  $$\sinh \left(z^{-1}\right) \sim \frac{1}{2} \exp \left(z^{-1}\right) \quad \text { as } \quad z \rightarrow 0, \operatorname{Arg} z \in(-\pi / 2, \pi / 2),$$

  and find the corresponding result in the sector $\operatorname{Arg} z \in(\pi / 2,3 \pi / 2)$.

  What is meant by the asymptotic expansion

  $$f(z) \sim \sum_{j=0}^{\infty} c_{j}\left(z-z_{0}\right)^{j} \quad \text { as } \quad z \rightarrow z_{0}, \operatorname{Arg}\left(z-z_{0}\right) \in\left(\theta_{0}, \theta_{1}\right) ?$$

  Show that the coefficients $\left\{c_{j}\right\}_{j=0}^{\infty}$ are determined uniquely by $f$. Show that if $f$ is analytic at $z_{0}$, then its Taylor series is an asymptotic expansion for $f$ as $z \rightarrow z_{0}\left(\right.$ for any $\left.\operatorname{Arg}\left(z-z_{0}\right)\right)$.

  Show that

  $$u(x, t)=\int_{-\infty}^{\infty} \exp \left(-i k^{2} t+i k x\right) f(k) d k$$

  defines a solution of the equation $i \partial_{t} u+\partial_{x}^{2} u=0$ for any smooth and rapidly decreasing function $f$. Use the method of stationary phase to calculate the leading-order behaviour of $u(\lambda t, t)$ as $t \rightarrow+\infty$, for fixed $\lambda$.

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Automata and Formal Languages

• Paper 2, Section $\mathbf{I}$, $4 F$

  Automata and Formal Languages | Part II, 2016

  (a) Which of the following are regular languages? Justify your answers.

  (i) $\left\{w \in\{a, b\}^{*} \mid w\right.$ is a nonempty string of alternating $a$ 's and $b$ 's $\}$.

  (ii) $\left\{w a b w \mid w \in\{a, b\}^{*}\right\}$.

  (b) Write down a nondeterministic finite-state automaton with $\epsilon$-transitions which accepts the language given by the regular expression $(\mathbf{a}+\mathbf{b})^{*}(\mathbf{b} \mathbf{b}+\mathbf{a}) \mathbf{b}$. Describe in words what this language is.

  (c) Is the following language regular? Justify your answer.

  $$\left\{w \in\{a, b\}^{*} \mid w \text { does not end in } a b \text { or } b b b\right\}$$

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Classical Dynamics

• Paper 2, Section I, E

  Classical Dynamics | Part II, 2016

  Consider the Lagrangian

  $$L=A\left(\dot{\theta}^{2}+\dot{\phi}^{2} \sin ^{2} \theta\right)+B(\dot{\psi}+\dot{\phi} \cos \theta)^{2}-C(\cos \theta)^{k}$$

  where $A, B, C$ are positive constants and $k$ is a positive integer. Find three conserved quantities and show that $u=\cos \theta$ satisfies

  $$\dot{u}^{2}=f(u)$$

  where $f(u)$ is a polynomial of degree $k+2$ which should be determined.

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• Paper 2, Section II, E

  Classical Dynamics | Part II, 2016

  Define what it means for the transformation $\mathbb{R}^{2 n} \rightarrow \mathbb{R}^{2 n}$ given by

  $$\left(q_{i}, p_{i}\right) \mapsto\left(Q_{i}\left(q_{j}, p_{j}\right), P_{i}\left(q_{j}, p_{j}\right)\right), \quad i, j=1, \ldots, n$$

  to be canonical. Show that a transformation is canonical if and only if

  $$\left\{Q_{i}, Q_{j}\right\}=0, \quad\left\{P_{i}, P_{j}\right\}=0, \quad\left\{Q_{i}, P_{j}\right\}=\delta_{i j}$$

  Show that the transformation $\mathbb{R}^{2} \rightarrow \mathbb{R}^{2}$ given by

  $$Q=q \cos \epsilon-p \sin \epsilon, \quad P=q \sin \epsilon+p \cos \epsilon$$

  is canonical for any real constant $\epsilon$. Find the corresponding generating function.

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Coding and Cryptography

• Paper 2, Section I, G

  Coding and Cryptography | Part II, 2016

  Show that the binary channel with channel matrix

  $$\left(\begin{array}{ll} 1 & 0 \\ \frac{1}{2} & \frac{1}{2} \end{array}\right)$$

  has capacity $\log 5-2$.

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• Paper 2, Section II, G

  Coding and Cryptography | Part II, 2016

  Define a $B C H$ code of length $n$, where $n$ is odd, over the field of 2 elements with design distance $\delta$. Show that the minimum weight of such a code is at least $\delta$. [Results about the Vandermonde determinant may be quoted without proof, provided they are stated clearly.]

  Let $\omega \in \mathbb{F}_{16}$ be a root of $X^{4}+X+1$. Let $C$ be the $\mathrm{BCH}$ code of length 15 with defining set $\left\{\omega, \omega^{2}, \omega^{3}, \omega^{4}\right\}$. Find the generator polynomial of $C$ and the rank of $C$. Determine the error positions of the following received words:

  (i) $r(X)=1+X^{6}+X^{7}+X^{8}$,

  (ii) $r(X)=1+X+X^{4}+X^{5}+X^{6}+X^{9}$.

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Cosmology

• Paper 2, Section I, C

  Cosmology | Part II, 2016

  A spherical cloud of mass $M$ has radius $r(t)$ and initial radius $r(0)=R$. It contains material with uniform mass density $\rho(t)$, and zero pressure. Ignoring the cosmological constant, show that if it is initially at rest at $t=0$ and the subsequent gravitational collapse is governed by Newton's law $\ddot{r}=-G M / r^{2}$, then

  $$\dot{r}^{2}=2 G M\left(\frac{1}{r}-\frac{1}{R}\right) .$$

  Suppose $r$ is given parametrically by

  $$r=R \cos ^{2} \theta$$

  where $\theta=0$ at $t=0$. Derive a relation between $\theta$ and $t$ and hence show that the cloud collapses to radius $r=0$ at

  $$t=\sqrt{\frac{3 \pi}{32 G \rho_{0}}},$$

  where $\rho_{0}$ is the initial mass density of the cloud.

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Differential Geometry

• Paper 2, Section II, G

  Differential Geometry | Part II, 2016

  If an embedded surface $S \subset \mathbf{R}^{3}$ contains a line $L$, show that the Gaussian curvature is non-positive at each point of $L$. Give an example where the Gaussian curvature is zero at each point of $L$.

  Consider the helicoid $S$ given as the image of $\mathbf{R}^{2}$ in $\mathbf{R}^{3}$ under the map

  $$\phi(u, v)=(\sinh v \cos u, \sinh v \sin u, u) .$$

  What is the image of the corresponding Gauss map? Show that the Gaussian curvature at a point $\phi(u, v) \in S$ is given by $-1 / \cosh ^{4} v$, and hence is strictly negative everywhere. Show moreover that there is a line in $S$ passing through any point of $S$.

  [General results concerning the first and second fundamental forms on an oriented embedded surface $S \subset \mathbf{R}^{3}$ and the Gauss map may be used without proof in this question.]

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Dynamical Systems

• Paper 2, Section II, E

  Dynamical Systems | Part II, 2016

  Consider the nonlinear oscillator

  $$\begin{aligned} \dot{x} &=y-\mu x\left(\frac{1}{2}|x|-1\right), \\ \dot{y} &=-x \end{aligned}$$

  (a) Use the Hamiltonian for $\mu=0$ to find a constraint on the size of the domain of stability of the origin when $\mu<0$.

  (b) Assume that given $\mu>0$ there exists an $R$ such that all trajectories eventually remain within the region $|\mathbf{x}| \leqslant R$. Show that there must be a limit cycle, stating carefully any result that you use. [You need not show that there is only one periodic orbit.]

  (c) Use the energy-balance method to find the approximate amplitude of the limit cycle for $0<\mu \ll 1$.

  (d) Find the approximate shape of the limit cycle for $\mu \gg 1$, and calculate the leading-order approximation to its period.

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Fluid Dynamics II

• Paper 2, Section II, B

  Fluid Dynamics II | Part II, 2016

  For a two-dimensional flow in plane polar coordinates $(r, \theta)$, state the relationship between the streamfunction $\psi(r, \theta)$ and the flow components $u_{r}$ and $u_{\theta}$. Show that the vorticity $\omega$ is given by $\omega=-\nabla^{2} \psi$, and deduce that the streamfunction for a steady two-dimensional Stokes flow satisfies the biharmonic equation

  $$\nabla^{4} \psi=0$$

  A rigid stationary circular disk of radius $a$ occupies the region $r \leqslant a$. The flow far from the disk tends to a steady straining flow $\mathbf{u}_{\infty}=(-E x, E y)$, where $E$ is a constant. Inertial forces may be neglected. Calculate the streamfunction, $\psi_{\infty}(r, \theta)$, for the far-field flow.

  By making an appropriate assumption about its dependence on $\theta$, find the streamfunction $\psi$ for the flow around the disk, and deduce the flow components, $u_{r}(r, \theta)$ and $u_{\theta}(r, \theta)$.

  Calculate the tangential surface stress, $\sigma_{r \theta}$, acting on the boundary of the disk.

  $[$ Hints: In plane polar coordinates $(r, \theta)$,

  $$\begin{gathered} \boldsymbol{\nabla} \cdot \mathbf{u}=\frac{1}{r} \frac{\partial\left(r u_{r}\right)}{\partial r}+\frac{1}{r} \frac{\partial u_{\theta}}{\partial \theta}, \quad \omega=\frac{1}{r} \frac{\partial\left(r u_{\theta}\right)}{\partial r}-\frac{1}{r} \frac{\partial u_{r}}{\partial \theta} \\ \nabla^{2} V=\frac{1}{r} \frac{\partial}{\partial r}\left(r \frac{\partial V}{\partial r}\right)+\frac{1}{r^{2}} \frac{\partial^{2} V}{\partial \theta^{2}}, \quad e_{r \theta}=\frac{1}{2}\left(r \frac{\partial}{\partial r}\left(\frac{u_{\theta}}{r}\right)+\frac{1}{r} \frac{\partial u_{r}}{\partial \theta}\right) \end{gathered}$$

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Further Complex Methods

• Paper 2, Section I, A

  Further Complex Methods | Part II, 2016

  The Euler product formula for the Gamma function is

  $$\Gamma(z)=\lim _{n \rightarrow \infty} \frac{n ! n^{z}}{z(z+1) \ldots(z+n)}$$

  Use this to show that

  $$\frac{\Gamma(2 z)}{2^{2 z} \Gamma(z) \Gamma\left(z+\frac{1}{2}\right)}=c,$$

  where $c$ is a constant, independent of $z$. Find the value of $c$.

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• Paper 2, Section II, A

  Further Complex Methods | Part II, 2016

  The Hurwitz zeta function $\zeta_{\mathrm{H}}(s, q)$ is defined for $\operatorname{Re}(q)>0$ by

  $$\zeta_{\mathrm{H}}(s, q)=\sum_{n=0}^{\infty} \frac{1}{(q+n)^{s}}$$

  State without proof the complex values of $s$ for which this series converges.

  Consider the integral

  $$I(s, q)=\frac{\Gamma(1-s)}{2 \pi i} \int_{\mathcal{C}} d z \frac{z^{s-1} e^{q z}}{1-e^{z}}$$

  where $\mathcal{C}$ is the Hankel contour. Show that $I(s, q)$ provides an analytic continuation of the Hurwitz zeta function for all $s \neq 1$. Include in your account a careful discussion of removable singularities. [Hint: $\Gamma(s) \Gamma(1-s)=\pi / \sin (\pi s)$.]

  Show that $I(s, q)$ has a simple pole at $s=1$ and find its residue.

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Galois Theory

• Paper 2, Section II, H

  Galois Theory | Part II, 2016

  (a) Let $K \subseteq L$ be a finite separable field extension. Show that there exist only finitely many intermediate fields $K \subseteq F \subseteq L$.

  (b) Define what is meant by a normal extension. Is $\mathbb{Q} \subseteq \mathbb{Q}(\sqrt{1+\sqrt{7}})$ a normal extension? Justify your answer.

  (c) Prove Artin's lemma, which states: if $K \subseteq L$ is a field extension, $H$ is a finite subgroup of $\operatorname{Aut}_{K}(L)$, and $F:=L^{H}$ is the fixed field of $H$, then $F \subseteq L$ is a Galois extension with $\operatorname{Gal}(L / F)=H$.

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General Relativity

• Paper 2, Section II, D

  General Relativity | Part II, 2016

  The Kasner (vacuum) cosmological model is defined by the line element

  $$d s^{2}=-c^{2} d t^{2}+t^{2 p_{1}} d x^{2}+t^{2 p_{2}} d y^{2}+t^{2 p_{3}} d z^{2} \quad \text { with } \quad t>0$$

  where $p_{1}, p_{2}, p_{3}$ are constants with $p_{1}+p_{2}+p_{3}=p_{1}^{2}+p_{2}^{2}+p_{3}^{2}=1$ and $0<p_{1}<1$. Show that $p_{2} p_{3}<0$.

  Write down four equations that determine the null geodesics of the Kasner model.

  If $k^{a}$ is the tangent vector to the trajectory of a photon and $u^{a}$ is the four-velocity of a comoving observer (i.e., an observer at rest in the $(t, x, y, z)$ coordinate system above), what is the physical interpretation of $k_{a} u^{a}$ ?

  Let $O$ be a comoving observer at the origin, $x=y=z=0$, and let $S$ be a comoving source of photons located on one of the spatial coordinate axes.

  (i) Show that photons emitted by $S$ and observed by $O$ can be either redshifted or blue-shifted, depending on the location of $S$.

  (ii) Given any fixed time $t=T$, show that there are locations for $S$ on each coordinate axis from which no photons reach $O$ for $t \leqslant T$.

  Now suppose that $p_{1}=1$ and $p_{2}=p_{3}=0$. Does the property in (ii) still hold?

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Graph Theory

• Paper 2, Section II, G

  Graph Theory | Part II, 2016

  Define the Turán graph $T_{r}(n)$, where $r$ and $n$ are positive integers with $n \geqslant r$. For which $r$ and $n$ is $T_{r}(n)$ regular? For which $r$ and $n$ does $T_{r}(n)$ contain $T_{4}(8)$ as a subgraph?

  State and prove Turán's theorem.

  Let $x_{1}, \ldots, x_{n}$ be unit vectors in the plane. Prove that the number of pairs $i<j$ for which $x_{i}+x_{j}$ has length less than 1 is at most $\left\lfloor n^{2} / 4\right\rfloor$.

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Integrable Systems

• Paper 2, Section II, D

  Integrable Systems | Part II, 2016

  What does it mean for $g^{\epsilon}:(x, u) \mapsto(\tilde{x}, \tilde{u})$ to describe a 1-parameter group of transformations? Explain how to compute the vector field

  $$V=\xi(x, u) \frac{\partial}{\partial x}+\eta(x, u) \frac{\partial}{\partial u}$$

  that generates such a 1-parameter group of transformations.

  Suppose now $u=u(x)$. Define the $n$th prolongation, $\mathrm{pr}^{(n)} g^{\epsilon}$, of $g^{\epsilon}$ and the vector field which generates it. If $V$ is defined by $(*)$ show that

  $$\mathrm{pr}^{(n)} V=V+\sum_{k=1}^{n} \eta_{k} \frac{\partial}{\partial u^{(k)}}$$

  where $u^{(k)}=\mathrm{d}^{k} u / \mathrm{d} x^{k}$ and $\eta_{k}$ are functions to be determined.

  The curvature of the curve $u=u(x)$ in the $(x, u)$-plane is given by

  $$\kappa=\frac{u_{x x}}{\left(1+u_{x}^{2}\right)^{3 / 2}}$$

  Rotations in the $(x, u)$-plane are generated by the vector field

  $$W=x \frac{\partial}{\partial u}-u \frac{\partial}{\partial x}$$

  Show that the curvature $\kappa$ at a point along a plane curve is invariant under such rotations. Find two further transformations that leave $\kappa$ invariant.

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Linear Analysis

• Paper 2, Section II, I

  Linear Analysis | Part II, 2016

  (a) Let $K$ be a topological space and let $C_{\mathbb{R}}(K)$ denote the normed vector space of bounded continuous real-valued functions on $K$ with the norm $\|f\|_{C_{\mathbb{R}}(K)}=\sup _{x \in K}|f(x)|$. Define the terms uniformly bounded, equicontinuous and relatively compact as applied to subsets $S \subset C_{\mathbb{R}}(K)$.

  (b) The Arzela-Ascoli theorem [which you need not prove] states in particular that if $K$ is compact and $S \subset C_{\mathbb{R}}(K)$ is uniformly bounded and equicontinuous, then $S$ is relatively compact. Show by examples that each of the compactness of $K$, uniform boundedness of $S$, and equicontinuity of $S$ are necessary conditions for this conclusion.

  (c) Let $L$ be a topological space. Assume that there exists a sequence of compact subsets $K_{n}$ of $L$ such that $K_{1} \subset K_{2} \subset K_{3} \subset \cdots \subset L$ and $\bigcup_{n=1}^{\infty} K_{n}=L$. Suppose $S \subset C_{\mathbb{R}}(L)$ is uniformly bounded and equicontinuous and moreover satisfies the condition that, for every $\epsilon>0$, there exists $n \in \mathbb{N}$ such that $|f(x)|<\epsilon$ for every $x \in L \backslash K_{n}$ and for every $f \in S$. Show that $S$ is relatively compact.

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Logic and Set Theory

• Paper 2, Section II, F

  Logic and Set Theory | Part II, 2016

  Define the von Neumann hierarchy of sets $V_{\alpha}$, and show that each $V_{\alpha}$ is a transitive set. Explain what is meant by saying that a binary relation on a set is well-founded and extensional. State Mostowski's Theorem.

  Let $r$ be the binary relation on $\omega$ defined by: $\langle m, n\rangle \in r$ if and only if $2^{m}$ appears in the base-2 expansion of $n$ (i.e., the unique expression for $n$ as a sum of distinct powers of 2 ). Show that $r$ is well-founded and extensional. To which transitive set is $(\omega, r)$ isomorphic? Justify your answer.

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Mathematical Biology

• Paper 2, Section I, B

  Mathematical Biology | Part II, 2016

  (a) The populations of two competing species satisfy

  $$\begin{aligned} \frac{d N_{1}}{d t} &=N_{1}\left[b_{1}-\lambda\left(N_{1}+N_{2}\right)\right] \\ \frac{d N_{2}}{d t} &=N_{2}\left[b_{2}-\lambda\left(N_{1}+N_{2}\right)\right] \end{aligned}$$

  where $b_{1}>b_{2}>0$ and $\lambda>0$. Sketch the phase diagram (limiting attention to $\left.N_{1}, N_{2} \geqslant 0\right)$.

  The relative abundance of species 1 is defined by $U=N_{1} /\left(N_{1}+N_{2}\right)$. Show that

  $$\frac{d U}{d t}=A U(1-U)$$

  where $A$ is a constant that should be determined.

  (b) Consider the spatial system

  $$\frac{\partial u}{\partial t}=u(1-u)+D \frac{\partial^{2} u}{\partial x^{2}}$$

  and consider a travelling-wave solution of the form $u(x, t)=f(x-c t)$ representing one species $(u=1)$ invading territory previously occupied by another species $(u=0)$. By linearising near the front of the invasion, show that the wave speed is given by $c=2 \sqrt{D}$.

  [You may assume that the solution to the full nonlinear system will settle to the slowest possible linear wave speed.]

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Number Fields

• Paper 2, Section II, F

  Number Fields | Part II, 2016

  (a) Prove that $5+2 \sqrt{6}$ is a fundamental unit in $\mathbb{Q}(\sqrt{6})$. [You may not assume the continued fraction algorithm.]

  (b) Determine the ideal class group of $\mathbb{Q}(\sqrt{-55})$.

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Number Theory

• Paper 2, Section I, I

  Number Theory | Part II, 2016

  Define the Legendre symbol and the Jacobi symbol. Compute the Jacobi symbols $\left(\frac{202}{11189}\right)$ and $\left(\frac{974}{1001}\right)$, stating clearly any properties of these symbols that you use.

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Numerical Analysis

• Paper 2, Section II, B

  Numerical Analysis | Part II, 2016

  (a) The advection equation

  $$u_{t}=u_{x}, \quad 0 \leqslant x \leqslant 1, t \geqslant 0$$

  is discretised using an equidistant grid with stepsizes $\Delta x=h$ and $\Delta t=k$. The spatial derivatives are approximated with central differences and the resulting ODEs are approximated with the trapezoidal rule. Write down the relevant difference equation for determining $\left(u_{m}^{n+1}\right)$ from $\left(u_{m}^{n}\right)$. What is the name of this scheme? What is the local truncation error?

  The boundary condition is periodic, $u(0, t)=u(1, t)$. Explain briefly how to write the discretised scheme in the form $B \mathbf{u}^{n+1}=C \mathbf{u}^{n}$, where the matrices $B$ and $C$, to be identified, have a circulant form. Using matrix analysis, find the range of $\mu=\Delta t / \Delta x$ for which the scheme is stable. [Standard results may be used without proof if quoted carefully.]

  [Hint: An $n \times n$ circulant matrix has the form

  $$A=\left(\begin{array}{cccc} a_{0} & a_{1} & \cdots & a_{n-1} \\ a_{n-1} & \ddots & \ddots & \vdots \\ \vdots & \ddots & \ddots & a_{1} \\ a_{1} & \cdots & a_{n-1} & a_{0} \end{array}\right)$$

  All such matrices have the same set of eigenvectors $\mathbf{v}_{\ell}=\left(\omega^{j \ell}\right)_{j=0}^{n-1}, \ell=0,1, \ldots, n-1$, where $\omega=e^{2 \pi i / n}$, and the corresponding eigenvalues are $\lambda_{\ell}=\sum_{k=0}^{n-1} a_{k} \omega^{k \ell}$.]

  (b) Consider the advection equation on the unit square

  $$u_{t}=a u_{x}+b u_{y}, \quad 0 \leqslant x, y \leqslant 1, t \geqslant 0$$

  where $u$ satisfies doubly periodic boundary conditions, $u(0, y)=u(1, y), u(x, 0)=u(x, 1)$, and $a(x, y)$ and $b(x, y)$ are given doubly periodic functions. The system is discretised with the Crank-Nicolson scheme, with central differences for the space derivatives, using an equidistant grid with stepsizes $\Delta x=\Delta y=h$ and $\Delta t=k$. Write down the relevant difference equation, and show how to write the scheme in the form

  $$\mathbf{u}^{n+1}=\left(I-\frac{1}{4} \mu A\right)^{-1}\left(I+\frac{1}{4} \mu A\right) \mathbf{u}^{n}$$

  where the matrix $A$ should be identified. Describe how (*) can be approximated by Strang splitting, and explain the advantages of doing so.

  [Hint: Inversion of the matrix $B$ in part (a) has a similar computational cost to that of a tridiagonal matrix.]

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Optimization and Control

• Paper 2, Section II, K

  Optimization and Control | Part II, 2016

  Consider a Markov decision problem with finite state space $X$, value function $F$ and dynamic programming equation $F=\mathcal{L} F$, where

  $$(\mathcal{L} \phi)(i)=\min _{a \in\{0,1\}}\left\{c(i, a)+\beta \sum_{j \in X} P_{i j}(a) \phi(j)\right\} .$$

  Suppose $0<\beta<1$, and $|c(i, a)| \leqslant B$ for all $i \in X, a \in\{0,1\}$. Prove there exists a deterministic stationary Markov policy that is optimal, explaining what the italicised words mean.

  Let $F_{n}=\mathcal{L}^{n} F_{0}$, where $F_{0}=0$, and $M_{n}=\max _{i \in X}\left|F(i)-F_{n}(i)\right|$. Prove that

  $$M_{n} \leqslant \beta M_{n-1} \leqslant \beta^{n} B /(1-\beta) .$$

  Deduce that the value iteration algorithm converges to an optimal policy in a finite number of iterations.

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Principles of Quantum Mechanics

• Paper 2, Section II, A

  Principles of Quantum Mechanics | Part II, 2016

  (a) Let $|j m\rangle$ be standard, normalised angular momentum eigenstates with labels specifying eigenvalues for $\mathbf{J}^{2}$ and $J_{3}$. Taking units in which $\hbar=1$,

  $$J_{\pm}|j m\rangle=\{(j \mp m)(j \pm m+1)\}^{1 / 2}|j m \pm 1\rangle .$$

  Check the coefficients above by computing norms of states, quoting any angular momentum commutation relations that you require.

  (b) Two particles, each of spin $s>0$, have combined spin states $|J M\rangle$. Find expressions for all such states with $M=2 s-1$ in terms of product states.

  (c) Suppose that the particles in part (b) move about their centre of mass with a spatial wavefunction that is a spherically symmetric function of relative position. If the particles are identical, what spin states $|J 2 s-1\rangle$ are allowed? Justify your answer.

  (d) Now consider two particles of spin 1 that are not identical and are both at rest. If the 3-component of the spin of each particle is zero, what is the probability that their total, combined spin is zero?

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Principles of Statistics

• Paper 2, Section II,

  Principles of Statistics | Part II, 2016

  (a) State and prove the Cramér-Rao inequality in a parametric model $\{f(\theta): \theta \in \Theta\}$, where $\Theta \subseteq \mathbb{R}$. [Necessary regularity conditions on the model need not be specified.]

  (b) Let $X_{1}, \ldots, X_{n}$ be i.i.d. Poisson random variables with unknown parameter $E X_{1}=\theta>0$. For $\bar{X}_{n}=(1 / n) \sum_{i=1}^{n} X_{i}$ and $S^{2}=(n-1)^{-1} \sum_{i=1}^{n}\left(X_{i}-\bar{X}_{n}\right)^{2}$ define

  $$T_{\alpha}=\alpha \bar{X}_{n}+(1-\alpha) S^{2}, \quad 0 \leqslant \alpha \leqslant 1$$

  Show that $\operatorname{Var}_{\theta}\left(T_{\alpha}\right) \geqslant \operatorname{Var}_{\theta}\left(\bar{X}_{n}\right)$ for all values of $\alpha, \theta$.

  Now suppose $\tilde{\theta}=\tilde{\theta}\left(X_{1}, \ldots, X_{n}\right)$ is an estimator of $\theta$ with possibly nonzero bias $B(\theta)=E_{\theta} \tilde{\theta}-\theta$. Suppose the function $B$ is monotone increasing on $(0, \infty)$. Prove that the mean-squared errors satisfy

  $$E_{\theta}\left(\tilde{\theta}_{n}-\theta\right)^{2} \geqslant E_{\theta}\left(\bar{X}_{n}-\theta\right)^{2} \text { for all } \theta \in \Theta$$

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Probability and Measure

• Paper 2, Section II, J

  Probability and Measure | Part II, 2016

  (a) State Jensen's inequality. Give the definition of $\|\cdot\|_{L^{p}}$ and the space $L^{p}$ for $1<p<\infty$. If $\|f-g\|_{L^{p}}=0$, is it true that $f=g$ ? Justify your answer. State and prove Hölder's inequality using Jensen's inequality.

  (b) Suppose that $(E, \mathcal{E}, \mu)$ is a finite measure space. Show that if $1<q<p$ and $f \in L^{p}(E)$ then $f \in L^{q}(E)$. Give the definition of $\|\cdot\|_{L^{\infty}}$ and show that $\|f\|_{L^{p}} \rightarrow\|f\|_{L^{\infty}}$ as $p \rightarrow \infty$.

  (c) Suppose that $1<q<p<\infty$. Show that if $f$ belongs to both $L^{p}(\mathbb{R})$ and $L^{q}(\mathbb{R})$, then $f \in L^{r}(\mathbb{R})$ for any $r \in[q, p]$. If $f \in L^{p}(\mathbb{R})$, must we have $f \in L^{q}(\mathbb{R})$ ? Give a proof or a counterexample.

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Representation Theory

• Paper 2, Section II, I

  Representation Theory | Part II, 2016

  Show that the 1-dimensional (complex) characters of a finite group $G$ form a group under pointwise multiplication. Denote this group by $\widehat{G}$. Show that if $g \in G$, the map $\chi \mapsto \chi(g)$ from $\widehat{G}$to $\mathbb{C}$ is a character of $\widehat{G}$, hence an element of $\widehat{G}$. What is the kernel of the $\operatorname{map} G \rightarrow \widehat{\widehat{G}}$?

  Show that if $G$ is abelian the map $G \rightarrow \widehat{\widehat{G}}$is an isomorphism. Deduce, from the structure theorem for finite abelian groups, that the groups $G$ and $\widehat{G}$are isomorphic as abstract groups.

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Riemann Surfaces

• Paper 2, Section II, H

  Riemann Surfaces | Part II, 2016

  Suppose that $f: \mathbb{C} / \Lambda_{1} \rightarrow \mathbb{C} / \Lambda_{2}$ is a holomorphic map of complex tori, and let $\pi_{j}$ denote the projection map $\mathbb{C} \rightarrow \mathbb{C} / \Lambda_{j}$ for $j=1,2$. Show that there is a holomorphic map $F: \mathbb{C} \rightarrow \mathbb{C}$ such that $\pi_{2} F=f \pi_{1} .$

  Prove that $F(z)=\lambda z+\mu$ for some $\lambda, \mu \in \mathbb{C}$. Hence deduce that two complex tori $\mathbb{C} / \Lambda_{1}$ and $\mathbb{C} / \Lambda_{2}$ are conformally equivalent if and only if the lattices are related by $\Lambda_{2}=\lambda \Lambda_{1}$ for some $\lambda \in \mathbb{C}^{*}$.

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Statistical Modelling

• Paper 2, Section I, K

  Statistical Modelling | Part II, 2016

  Define an exponential dispersion family. Prove that the range of the natural parameter, $\Theta$, is an open interval. Derive the mean and variance as a function of the log normalizing constant.

  [Hint: Use the convexity of $e^{x}$, i.e. $e^{p x+(1-p) y} \leqslant p e^{x}+(1-p) e^{y}$ for all $\left.p \in[0,1] .\right]$

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Statistical Physics

• Paper 2, Section II, C

  Statistical Physics | Part II, 2016

  (a) What is meant by the canonical ensemble? Consider a system in the canonical ensemble that can be in states $|n\rangle, n=0,1,2, \ldots$ with energies $E_{n}$. Write down the partition function for this system and the probability $p(n)$ that the system is in state $|n\rangle$. Derive an expression for the average energy $\langle E\rangle$ in terms of the partition function.

  (b) Consider an anharmonic oscillator with energy levels

  $$\hbar \omega\left[\left(n+\frac{1}{2}\right)+\delta\left(n+\frac{1}{2}\right)^{2}\right], \quad n=0,1,2, \ldots$$

  where $\omega$ is a positive constant and $0<\delta \ll 1$ is a small constant. Let the oscillator be in contact with a reservoir at temperature $T$. Show that, to linear order in $\delta$, the partition function $Z_{1}$ for the oscillator is given by

  $$Z_{1}=\frac{c_{1}}{\sinh \frac{x}{2}}\left[1+\delta c_{2} x\left(1+\frac{2}{\sinh ^{2} \frac{x}{2}}\right)\right], \quad x=\frac{\hbar \omega}{k_{B} T}$$

  where $c_{1}$ and $c_{2}$ are constants to be determined. Also show that, to linear order in $\delta$, the average energy of a system of $N$ uncoupled oscillators of this type is given by

  $$\langle E\rangle=\frac{N \hbar \omega}{2}\left\{c_{3} \operatorname{coth} \frac{x}{2}+\delta\left[c_{4}+\frac{c_{5}}{\sinh ^{2} \frac{x}{2}}\left(1-x \operatorname{coth} \frac{x}{2}\right)\right]\right\}$$

  where $c_{3}, c_{4}, c_{5}$ are constants to be determined.

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Stochastic Financial Models

• Paper 2, Section II, K

  Stochastic Financial Models | Part II, 2016

  In the context of the Black-Scholes model, let $S_{0}$ be the initial price of the stock, and let $\sigma$ be its volatility. Assume that the risk-free interest rate is zero and the stock pays no dividends. Let $\operatorname{EC}\left(S_{0}, K, \sigma, T\right)$ denote the initial price of a European call option with strike $K$ and maturity date $T$.

  (a) Show that the Black-Scholes formula can be written in the form

  $$\mathrm{EC}\left(S_{0}, K, \sigma, T\right)=S_{0} \Phi\left(d_{1}\right)-K \Phi\left(d_{2}\right)$$

  where $d_{1}$ and $d_{2}$ depend on $S_{0}, K, \sigma$ and $T$, and $\Phi$ is the standard normal distribution function.

  (b) Let $\operatorname{EP}\left(S_{0}, K, \sigma, T\right)$ be the initial price of a put option with strike $K$ and maturity $T$. Show that

  $$\operatorname{EP}\left(S_{0}, K, \sigma, T\right)=\operatorname{EC}\left(S_{0}, K, \sigma, T\right)+K-S_{0}$$

  (c) Show that

  $$\operatorname{EP}\left(S_{0}, K, \sigma, T\right)=\operatorname{EC}\left(K, S_{0}, \sigma, T\right)$$

  (d) Consider a European contingent claim with maturity $T$ and payout

  $$S_{T} I_{\left\{S_{T} \leqslant K\right\}}-K I_{\left\{S_{T}>K\right\}}$$

  Assuming $K>S_{0}$, show that its initial price can be written as $\mathrm{EC}\left(S_{0}, K, \hat{\sigma}, T\right)$ for a volatility parameter $\hat{\sigma}$ which you should express in terms of $S_{0}, K, \sigma$ and $T$.

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Topics in Analysis

• Paper 2, Section I, H

  Topics in Analysis | Part II, 2016

  Define what it means for a subset $E$ of $\mathbb{R}^{n}$ to be convex. Which of the following statements about a convex set $E$ in $\mathbb{R}^{n}$ (with the usual norm) are always true, and which are sometimes false? Give proofs or counterexamples as appropriate.

  (i) The closure of $E$ is convex.

  (ii) The interior of $E$ is convex.

  (iii) If $\alpha: \mathbb{R}^{n} \rightarrow \mathbb{R}^{n}$ is linear, then $\alpha(E)$ is convex.

  (iv) If $f: \mathbb{R}^{n} \rightarrow \mathbb{R}^{n}$ is continuous, then $f(E)$ is convex.

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• Paper 2, Section II, H

  Topics in Analysis | Part II, 2016

  Prove Bernstein's theorem, which states that if $f:[0,1] \rightarrow \mathbb{R}$ is continuous and

  $$f_{m}(t)=\sum_{r=0}^{m}\left(\begin{array}{c} m \\ r \end{array}\right) f(r / m) t^{r}(1-t)^{m-r}$$

  then $f_{m}(t) \rightarrow f(t)$ uniformly on $[0,1]$. [Theorems from probability theory may be used without proof provided they are clearly stated.]

  Deduce Weierstrass's theorem on polynomial approximation for any closed interval.

  Proving any results on Chebyshev polynomials that you need, show that, if $g:[0, \pi] \rightarrow \mathbb{R}$ is continuous and $\epsilon>0$, then we can find an $N \geqslant 0$ and $a_{j} \in \mathbb{R}$, for $0 \leqslant j \leqslant N$, such that

  $$\left|g(t)-\sum_{j=0}^{N} a_{j} \cos j t\right| \leqslant \epsilon$$

  for all $t \in[0, \pi]$. Deduce that $\int_{0}^{\pi} g(t) \cos n t d t \rightarrow 0$ as $n \rightarrow \infty$.

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Waves

• Paper 2, Section II, 37D

  Waves | Part II, 2016

  Starting from the equations for one-dimensional unsteady flow of a perfect gas at constant entropy, show that the Riemann invariants

  $$R_{\pm}=u \pm \frac{2\left(c-c_{0}\right)}{\gamma-1}$$

  are constant on characteristics $C_{\pm}$given by $d x / d t=u \pm c$, where $u(x, t)$ is the speed of the gas, $c(x, t)$ is the local speed of sound, $c_{0}$ is a constant and $\gamma>1$ is the exponent in the adiabatic equation of state for $p(\rho)$.

  At time $t=0$ the gas occupies $x>0$ and is at rest at uniform density $\rho_{0}$, pressure $p_{0}$ and sound speed $c_{0}$. For $t>0$, a piston initially at $x=0$ has position $x=X(t)$, where

  $$X(t)=-U_{0} t\left(1-\frac{t}{2 t_{0}}\right)$$

  and $U_{0}$ and $t_{0}$ are positive constants. For the case $0<U_{0}<2 c_{0} /(\gamma-1)$, sketch the piston path $x=X(t)$ and the $C_{+}$characteristics in $x \geqslant X(t)$ in the $(x, t)$-plane, and find the time and place at which a shock first forms in the gas.

  Do likewise for the case $U_{0}>2 c_{0} /(\gamma-1)$.

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