Toronto Math Forum
APM3462022S => APM346Lectures & Home Assignments => Chapter 2 => Topic started by: Zicheng Ding on January 31, 2022, 09:04:09 PM

I was a little confused about the region marked in red in this example. For the region on the right we get $\phi(x) = \sin(3x)$ and $\psi(x) = 3\sin(3x)$, and from the boundary conditions we can get $\phi(x) = \psi(x) = 0$ for $t > 0$, so we have $u = 0$ for the region on the left. For the middle region in red, since $\psi(x  \frac{t}{3})$ is undefined, we can just have it as $\psi(x) = 0$ instead? The middle region is like a mix of the other two regions so I am a little uncertain.

For the middle region in red, since $\psi(x  \frac{t}{3})$ is undefined,
It is defined, because on the line $\{x=t, t>0\}$ we have not 1 but 2 boundary conditions, so in the domain $\{x>t, t>0\}$ we have essentially a Cauchy problem with the date on the line consisting of two rays: $\{x>0,t=0\}$ and $\{x=t, t>0\}$.