Problem set 7, Harmonic Analysis

Due March 22

Problem 1

For any dimension $latex d\ge 1$,

$latex \displaystyle \int_{\mathbb R^d} \frac{dx}{(|x|^2 + 1)^{(d+1)/2}} = \frac{\pi^{(d+1)/2}}{\Gamma((d+1)/2)},$

and verify that

$latex \displaystyle \int_{\mathbb R^d} P_t(x) dx = 1.$

(Hint: Use spherical coordinates and the identity $latex \displaystyle \int_0^\infty t^\alpha e^{-ts} \frac{dt}{t} = \frac{\Gamma(\alpha)}{s^\alpha}$ for any $latex \alpha, s>0$.

Problem 2

Let $latex f$ be integrable on $latex \mathbb R^d$.

1. For any $latex h\in\mathbb R^d$, $latex \displaystyle \int_{\mathbb R^d} f(x) dx = \int_{\mathbb R^d} f(x - h) dx.$
2. For any $latex r>0$, $latex \displaystyle \int_{\mathbb R^d} f(x) dx = \frac{1}{r^d} \int_{\mathbb R^d} f\big( \frac{x}{r}\big) dx.$

Problem 3

If $latex f\in C_0(\mathbb R^d)$, then $latex u(x,t) \to f(x)$ as $latex t\to 0$, uniformly in $latex x\in\mathbb R^d$.

Problem 4

Let $latex f\in C_c(\mathbb R^d)$. Then $latex \displaystyle \int_{\mathbb R^d} |f(x - h) - f(x)| dx \to 0$ as $latex h\to 0$ in $latex \mathbb R^d$. (Hint: $latex f$ is uniformly continuous in its compact support.)

Problem 5 (Principle of subordination)

Let $latex H_t(x)$ be the heat kernel, $latex \displaystyle H_t(x) = \frac{1}{(4\pi t)^{d/2}} e^{-|x|^2/4t}$. Then 

$latex \displaystyle P_t(x) = \frac{t}{2\sqrt\pi} \int_0^\infty e^{-t^2/4s} H_s(x) \frac{ds}{s^{3/2}}.$

(Hint: use the identifty of the Gamma function as in Problem 1.)