Math 642-600 Assignments — Spring 2017
Assignment 1 - Due Wednesday, 1/25/2017.
- Do the following problems.
- Problem 2, page 204 (§ 5.1).
- Problem 7, page 204 (§ 5.1).
- Hanging chain problem. A chain having uniform linear
density ρ hangs between the points (0,0) and (1,0). The total mass
m, which is fixed, and the total energy E of the chain are
Assuming that the chain hangs in a shape that minimizes the
energy, find the shape of the hanging chain.
- Consider the functional J(y) =
∫abF(x,y,y′)dx, where y ∈
C1 and y(a)=A, y(b)=B are fixed.
- Let y extremize J, subject to all of the conditions above. Verify
that if η ∈ C1 and η(a)=η(b)=0,
then
Δ J = ∫ab (-g(x) +
Fy′(x,y,y′))η′(x)dx, where g(x) =
∫axFy(u,y,y′)du.
- Use (a) to show that -g+Fy′(x,y,y′)=c, a
constant. From this, it follows that
Fy′(x,y,y′)∈ C1. (Du
Bois-Reymond, 1879. The point is that one need not assume that
y′′ exists.)
- Smoothing spline. Let J[y] =
∫01 (y′(x))2dx. Let the
admissible set for J be all piecewise C1 curves on [0,1]
that satisfy y(k/n) = yk , for k = 0, 1, 2, ..., n, with
the discontinuities in y′ appearing only at the points
xk = k/n. Use
the previous problem to show that the minimizer for J is a linear
spline passing through all of the points {(xk,yk), k = 0,
..., n}. (Hint: y(x) is in C1[xk,
xk+1]).)
Assignment 2 - Due Wednesday, 2/1/2017.
- Do the following problems.
- Problem 5, page 204 (§ 5.1).
- Problem 9, page 204 (§ 5.1).
- Problem 12, page 204 (§ 5.1).
- Consider the torus (surface of a "doughnut") obtained by rotating
a circle with center $(R,0,0)$ and radius $r < R $ about the $z$
axis. Find the metric tensor for it and use this tensor to obtain
the 2nd order differential equations for the geodesics.
- Problem 6, page 205 (§ 5.2).
- Find the Legendre transformation H(p) for F(x) =
xT A x, where x is in
Rn and A is a symmetric, positive definite n×n
matrix.
Assignment 3 - Due Wednesday, 2/8/2016.
- Do the following problems.
- Problem 7, page 206 (§ 5.2). (See Fig. 5.4, p. 205 for a
diagram.)
- Problem 8, page 206 (§ 5.2). (Note: There is an error in the
expression for L. The (dθ/dt)2
sin2θ) should be replaced by
(dφ/dt)2 sin2θ.)
- The Hamiltonian of a system having a Lagrangian
$L(q_1,q_2,\ldots,q_n, \dot q_1,\dot q_2,\ldots,\dot q_n)$ is
defined as the Legendre transform of $L$ with respect to the $\dot
q_j$'s. (See pg. 187). In class we showed that, for a
planet (mass $m$) in orbit around the sun (mass=$M$), where the
constant $K=GmM$, the polar form of the Lagrangian is $
L=\frac{m}{2}\big(\dot r^2 + r^2 \dot \theta^2\big)+ Kr^{-1}$.
- Show that the Hamiltonian $H=\frac{1}{2m}
(p_r^2+r^{-2}p_\theta^2) - Kr^{-1}$.
- Show that the angular momentum $p_\theta=\ell$ is a constant of the
motion.
- Show that the Hamiltonian is a constant of
the motion, $H=E$, where $E$ is the total energy of the system.
- Show that $p_r = \ell r^{-2} \frac{dr}{d\theta} = -
\ell \frac{d}{d\theta}(r^{-1})$.
- Let $u = r^{-1}- mK\ell^{-2}$. Show that $2m E\ell^{-2} + m^2K^2\ell^{-4}
= \big(\frac{du}{d\theta}\big)^2 + u^2$. Solve this first order differential
equation for u in terms of $\theta$.
- Use your solution to the previous part to establish Kepler's laws
of planetary motion:
- The orbit of a planet is an ellipse, with the sun at one
focus.
- The radius vector from the sun to the planet sweeps out equal
areas in equal times.
- The square of the period of any planet is proportional to the
cube of the semimajor axis of its orbit. The proportionality constant
is the same for all planets.
Assignment 4 - Due Friday, 2/17/2017.
- Read sections 6.1 and 6.2.
- Do the following problems.
- Problem 1, page 205 (§ 5.2).
- Problem 3, page 205 (§ 5.2).
- Problem 4, page 205 (§ 5.2). (Attaching a mass at the
midpoint $x_c$ amounts to making changing the density from $\rho$ to
$\rho+ m\delta(x-x_c)$.)
- Problem 1, page 207 (§ 5.4). (Hint: Use Minimax Principle.)
- Use a quadratic polynomial to estimate the lowest eigenvalue for
the Sturm-Liouville problem with Lu = − u′′ =
λu, u(0) = 0, u′(1)+u(1) = 0.
- Problem 6, page 208 (§ 5.4). Take Ω to be a 2D disk
centered at 0 and having radius r = a. How does the lowest
eigenvalue change with a? Explain.
Assignment 5 - Due Monday, 2/27/2017.
- Read sections 6.2 and 6.4.
- Do the following problems.
- Do problems 4, 5 and 11 in
the
Exercises for Complex Variables
- Find all of the Laurent expansions about $z=0$ for $f(z) =
\frac{1}{(z^2-1)(z+2)}$
- Section 6.1: 4, 6
- Section 6.2: 6, 9, 10
- Section 6.4: 2, 4, 9
- Let $\xi \ge 0$. Find $\hat f(\xi) := \frac{1}{2\pi}\int_{-\infty}^\infty
\frac{e^{i\xi x}}{1+x^4}dx$.
Assignment 6 - Due Wednesday, 3/8/2017.
- Read section 6.5 and 7.1.
- Do the following problems.
- Section 6.4: 3, 7, 20, 25 (You may use the version of the residue
theorem proved in class on 2/27).
- Section 6.5: 2
- Suppose that $f(z)$ is analytic in and on a simple closed curve
$C$, except for isolated singularities at $z_1,z_2,\ldots,z_n$ and
one simple pole at a point $z_0$ on the boundary. If $z_0$ is on a
corner of the curve where the angle between the new and old
directions is $\alpha$, show that
\[
\oint_Cf(z)dz= 2\pi i\sum_{j=1}^n Res_{z_j}(f) + i\alpha \,Res_{z_0}(f)
\]
Assignment 7 - Due Monday, 4/10/2017.
- Read section 7.2.1.
- Do the following problems.
- Section 7.1: 2(a), 2(b). For each operator in 2(a), 2(b), find
the norm and adjoint $L^\ast$. You may use any of the problems below.
- Let $L$ be a bounded linear operator on $\mathcal H$; that is, $L
\in {\mathcal B}(\mathcal H)$. Show that $\rho(L)$ contains the
exterior of the disk $\{\lambda \in {\mathbb C} \colon |\lambda|
\le ||L||\}$. (Consequently, $\sigma(L)$ is contained in the disk.)
- Let $K \in {\mathcal B}(\mathcal H)$ and let $L:D \to \mathcal H$ be a
densely defined, closed linear operator.
- Show that $L+K$ is defined on $D$ and is also closed.
- Show that $(L+K)^\ast$ is defined on the domain of $L^\ast$ and
that $(L+K)^\ast = L^\ast + K^\ast$.
- Suppose that $0 \in \rho(L)$. Show that $0 \in \rho(L^\ast)$ and
that $(L^{-1})^\ast = (L^\ast)^{-1}$.
- Let $L = L^\ast$ be defined on a domain $D$. In class, we showed
that $\sigma_r(L) = \emptyset$ and that $\sigma_d(L) \subset \mathbb
R$. Show that $\sigma_c(L) \subseteq \mathbb R$. Thus, $\sigma(L)$ is
a subset of the reals.
- Let $L = L^\ast$ be defined on a domain $D$ and suppose that $L$
satisfies $\langle Lf,f \rangle \ge 0 \ $, for all $f\in D$. Show that
$\sigma(L) \subseteq [0, \infty)$.
Assignment 8 - Due Monday, 4/24/2017.
- Read section 7.2.1.
- Do the following problems.
- Let $L$ be a closed, densely defined operator on a Hilbert space
$\mathcal H$ and let $R_\lambda(L)$ be the resolvent operator. Prove this:
Let $D_L$ be the domain of $L$. Then, on $D_L$, $LR_\lambda =
R_\lambda L$.
- Suppose that $L\in \mathcal B(\mathcal H)$ and that $C$ is a
simple closed rectifiable curve in that encloses $\sigma(L)$. Let
$P_C := -\frac{1}{2\pi i}\oint_C (L-\lambda I)^{-1}d
\lambda= -\frac{1}{2\pi
i}\oint_C R_\lambda(L) d\lambda$.
- Show that $P_c=I$.
- Show that for every polynomial $p(\lambda)$ one has
\[ p(L) = -\frac{1}{2\pi i}\oint_C p(\lambda)R_\lambda(L) d\lambda \]
- Let $L=L^\ast$ and let $C$ be a positively oriented, simple
closed curve in the resolvent set, encircling $\sigma_0\subset
\sigma(L)\in \mathbb R$ but no other points in
$\sigma(L)$. (Note: the analyticity of $R_\lambda(L)$ implies that $C$
may be deformed into any contour with the same properties.) Let
$P_C := -\frac{1}{2\pi i}\oint_C R_\lambda(L) d\lambda$.
- Consider the curve $C'$ that is the complex conjugate of
$C$. Show that $C'$ contains $\sigma_0$, that it is negatively
oriented, and that $P_{-C'} =-\frac{1}{2\pi i}\oint_{-C'} R_\lambda
d\lambda=P_C$.
- Use part (a) to show that $P_C^\ast = P_C$. (Since we know $P_C$
is a projection, this implies that it is an orthogonal
projection.
- Let $L=A$ be a self-adjoint, $n\times n$ matrix. Suppose that $A$
has $r$ distinct eigenvalues, arranged in the order $\lambda_1 <
\lambda_2 < \cdots < \lambda_r$. In addition, let the orthogonal
projections onto the eigenspaces be $Q_1,\ldots,Q_r$. Suppose $a <
b$, and neither is an eigenvalue of $A$; let $C_\varepsilon$ be a
positively oriented rectangular contour, with corners
$a-i\varepsilon$, $b-i\varepsilon$, $b+i\varepsilon$, and
$a+i\varepsilon$. Show that \[ \lim_{\varepsilon\downarrow
0}\,-\frac{1}{2\pi i} \oint_{C_\varepsilon} (A-\lambda I)^{-1}d\lambda
= E_b-E_a, \] where $E_\lambda := \sum_{j=1}^r H(\lambda - \lambda_j)Q_j$
and $H(t)=\begin{cases} 1 & t\ge 0\\0 & t<0\end{cases}$.
- Let $\mathcal H$ be a Hilbert space and suppose that $E$ and $F$
are orthogonal projections satisfying $E\ge F$ i.e., $\langle
Ef,f\rangle \ge \langle Ff,f\rangle$. Show that $EF=FE = F$. (Hint:
Note that $\langle (E-F)f,f\rangle \ge 0$. Use $f=(I-E)g$, where $g\in
\mathcal H$.)
- Find the Green's function for the operator $Lu = -u''$, with
$D_L=\{u,u''\in L^2[0,\infty): u'(0)=0\}$.
Assignment 9 - Not to be handed in.
- Suggested problems (Fourier transform convention: $\hat f(\xi) =
\int_{-\infty}^\infty f(x)e^{-i\xi x}dx$.)
- Consider the Green's function that you found in problem 6,
Assignment 8, for the operator $Lu- -u''$, $D_L=\{u,u''\in
L^2[0,\infty)\colon u'(0)=0\}$. Use it and Stone's formula for the
spectral family of $L$ to derive the Fourier cosine transform pair:
\[
F(\xi)= \frac{2}{\pi}\int_0^\infty f(x)\cos(\xi x)dx \ \text{and}\
f(x)= \int_{0}^\infty F(\xi)\cos(\xi x)d\xi
\]
- Prove the following theorems:
-
If $f,g \in L^1(\mathbb R) \cap L^2(\mathbb R)$, then $f\ast g\in
L^1(\mathbb R) \cap L^\infty(\mathbb R)$.
- If $f\in L^1(\mathbb R)$, then its Fourier transform $\hat f$ is in
$C_0(\mathbb R)$.
- Consider the one dimensional heat equation, $u_t = u_{xx}$, with
$u(x,0) = f(x)$, where $-\infty < x < \infty$ and $0 < t <
\infty$. You are given that ${\mathcal F}[e^{-x^2}]
=\sqrt{\pi}e^{-\xi^2/4}$. By taking the Fourier transform in $x$,
show that the solution $u(x,t)$ is given by
\[
u(x,t) = \int_{\mathbb R}K(x-y,t)f(y)dy,\ K(\xi,t) =
(4\pi t)^{-1/2} e^{-\xi^2/4t}.
\]
(The function $K(\xi,t)$ is the one-dimensional heat kernel.)
- Let T be a tempered distribution. Find the Fourier transforms for
T′, xT(x), T(x−a), and eibxT(x).
- Let $f$ be in Schwartz space, $\mathcal S$, and let $g$ be $C^\infty$
and satisfy $|g^{(m)}(x)| \le c_m(1+x^2)^{n_m}$
for all nonnegative integers m. Here $c_m$ and $n_m$
depend on $g$ and $m$. Show that $fg$ is in Schwartz space. Use this
result to explain how to
define the product $g(x)T(x)$, where $T$ is a tempered distribution.
- Section 10.3: 1, 3, 14.
Updated 5/3/2017 (fjn)