# Ngô Quốc Anh

## February 26, 2010

### Wave equations: The first pointwise estimates

Filed under: Nghiên Cứu Khoa Học, PDEs — Tags: — Ngô Quốc Anh @ 19:15

Followed by the topic where we discuss a pointwise estimate of solution to a class of heat equations. We are now interested in the case of wave equations.

Consider the wave equation in $\mathbb R^3$

$\displaystyle u_{tt}-\Delta u =0, \quad x\in \mathbb R^3, t>0$

together with the following initial data

$\displaystyle u(x,0)=0, \quad u_t(x,0)=g(x)$.

We assume $g \in C_0^\infty(\mathbb R^3)$. We will prove the following estimate

$\displaystyle |u(x,t)| \leq \frac{C}{t}, \quad t>0$

for some constant $C$ depending only on the given data.

Proof. The assumption on $g$ implies the existence of two positive constants $R$ and $M$ such that

$\displaystyle {\rm supp}(g) \subset B_R = \{ x \in \mathbb R^3:|x|

and

$\displaystyle |g(x)| \leq M, \quad \forall x \in \mathbb R^3$.

From the Poisson formula (but is known as Kirchhoff formula) in 3D, the solution to the problem is given as the following

$\displaystyle u(x,t)=\frac{1}{4\pi t} \int_{|y-x|=t} {g(y)dS_y}$.

It is easy to verify that: the area of the intersection $\{ y \in \mathbb R^3:|y-x|=t\} \cap B_R$ is less than or equals to the area of $\partial B_R$. Therefore, we obtain

$\displaystyle |u(x,t)| \leq\frac{1}{4\pi t} M 4\pi R^2 = \frac{MR^2}{t}, \quad t>0$.