APM346-2022S > Chapter 2

Online textbook, Chapter 2.6, example 7

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**Zicheng Ding**:

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.

**Victor Ivrii**:

--- Quote from: Zicheng Ding on January 31, 2022, 09:04:09 PM --- For the middle region in red, since $\psi(x - \frac{t}{3})$ is undefined,

--- End quote ---

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 $\{x=-t, t>0\}$.

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