# Uniqueness of solutions to the wave equation

we are given the problem $u_{tt}-c^2\Delta u=0$ with conditions $u(x,0)=u_0(x)$ and $u_t(x,0)=u_1(x)$ where $x\in\mathbb{R}^n$ and $u_0,u_1\in\mathcal{C}^1$ having compact supports. If a solution exists, is it possible to conclude that this solution has a compact support, too? Thanks.

• The solution will not have global compact support for $t \in [0,\infty)$, but it will for every finite time because of finite wave propagation speed. Is that what you mean? – Lukas Geyer Jan 21 '15 at 15:48
• Thanks for your answer! Basically, I want to prove uniqueness of the solution if the intial data have a compact support. Therefore, my idea was to use energy conservation which my professor proved in the case when the solution has a compact support. Unfortunately, this seems not to be true in general and I have no nice tool such as D'Alembert's formula as in the one-dimensional case available. Is there another way to prove uniqueness of the solution having compactly supported data? – JohnSmith Jan 21 '15 at 23:35
• Hint: Consider two solutions say $u,v$ of your wave equation. The difference, $w$ will be a solution too, but with null data. Now write down the weak formulation (with null data) and use as test function $w_t$. Since you can write the term $(w_{tt},w_t)$ as $\frac{d}{2dt}(||w||^2_{L^2})$, Gronwall's inequality should provide you uniqueness. This is a very standard routine, see e.g. Evans, Partial Differential Equations for further details. – alemou Jan 22 '15 at 13:39

Both uniqueness and finite propagation speed follow by an energy method just like in the one-dimensional case. For the sake of simplicity the following is for $c=1$, the same argument works for arbitrary $c>0$.
Fix some point $x_0 \in \mathbb{R}^n$ and time $t_0 > 0$, and define $$E(t) = \int_{B(x_0, t_0-t)} \left( u_t(x,t)^2 + |\nabla u (x,t)|^2 \right) \, dx$$ for $0 \le t \le t_0$. Using integration by parts (Green's identities) you should get $$E'(t) = \int_{\partial B(x_0, t_0-t)} \left( 2 u_t \frac{\partial u}{\partial n} -u_t^2 - |\nabla u|^2\right) \, dx$$ Then, using the inequality $$\left| 2u_t \frac{\partial u}{\partial n} \right| \le 2|u_t| \, |\nabla u| \le u_t^2 + |\nabla u|^2$$ you get that $E'(t) \le 0$. So if you know $u(x,0)\equiv u_t(x,0)\equiv 0$ for $x \in B(x_0, t_0)$, then $E(0)=0$, and so $E(t)=0$ for $0 \le t \le t_0$, implying that $u_t(x,t) = 0$ and $\nabla u (x,t) = 0$ for $0\le t \le t_0$ and $|x-x_0| \le t_0 - t$, which implies $u(x,t) = 0$ for the same parameters. This directly implies uniqueness and propagation speed $\le 1$.