# Domain of influence for wave equation in 2 dimensions

1. Mar 26, 2009

### somethingstra

Hello, I have some trouble seeing why the solution of the wave equation in 2 dimensions exist at all later times once it passes an initial disturbance....

For example, take a simple case where the initial position is zero, and the initial velocity equals some function inside some circle domain. The solution would be:

$$\frac{1}{2\pi }\int \int \frac{\psi (x,y)\partial x\partial y}{\sqrt{t_{o}^{2}-(x-x_{0})^{2}-(y-y_{o})^{2}}}$$

1) Where in that equation tells you that the solutions continues to exist at all later times?
2) If the initial velocity was zero outside the circle domain, why would the solution continue to exist? If we plug in Ψ = 0, wouldn't the solution be zero instead?
3) Can a solution be negative?

2. Mar 27, 2009

### somethingstra

Maybe I am not being clear. What I just want to know is how this solution shows that it extends outwards at t>0 and why it continues to exist at all later times. In other words, can somebody prove to me why Hyugen's principle fails at dimension 2?

Last edited: Mar 27, 2009
3. Mar 28, 2009

### somethingstra

bump...two days and no answer at all?

4. Mar 29, 2009

### matematikawan

5. Mar 29, 2009

### somethingstra

I've read that many times and still do not understand. Can someone explain it from the formula I posted above?