MHB Why are there 2 cosines used in the general solution for this domain?

  • Thread starter Thread starter Dustinsfl
  • Start date Start date
  • Tags Tags
    Domain Laplace
Dustinsfl
Messages
2,217
Reaction score
5
The domain is \(0\leq x < \infty\) and \(0\leq y\leq b\).
\begin{align*}
\phi(x, 0) &= f(x)\\
\phi(x, b) &= 0
\end{align*}
I want to show that
\[
\phi(x, y) = \frac{1}{\pi}\int_0^{\infty} \int_{-\infty}^{\infty}f(\xi) \frac{\sinh[u(b - y)]}{\sinh(ub)} \cos[u(\xi - x)]d\xi du
\]
Why isn't the periodic function cosine used on y? How should this be started then?
 
Physics news on Phys.org
Question is at the bottom in red.
Our boundary conditions on \(x\) are
\[
\lim_{x\to\pm\infty}\phi(x, y) = 0.
\]
Let \(\phi(x, y) = \varphi(x)\psi(y)\).
Then \(\frac{\varphi''}{\varphi} = - \frac{\psi''}{\psi} = -k^2\).
\begin{align}
\varphi(x) &\sim\left\{\cos(kx), \sin(kx)\right\}\\
\psi(y) &\sim\left\{\cosh(ky), \sinh(ky)\right\}
\end{align}
From the boundary conditions on \(y\), we see that we need to make a change of
variables.
That is, \(y\to b - y^*\); however, since the choice of variables are
arbitrary, let the change of variable be \(b - y\).
So we have is
\[
\psi(y) \sim\left\{\cosh(k(b - y)), \sinh(k(b - y))\right\}.
\]
Using the boundary condition \(\phi(x, b) = 0\), we have that
\[
\psi(y) \sim\sinh(k(b - y)).
\]
The general solution is then
\begin{alignat}{2}
\phi(x, y) &= \int_0^{\infty}\left[A(k)\cos(kx) + B(k)\sin(kx)\right]
\sinh(k(b - y))dk\\
\phi(x, 0) &= \int_0^{\infty}\left[A(k)\cos(kx) + B(k)\sin(kx)\right]
\sinh(kb)dk && ={} f(x)
\end{alignat}
Let \(A^*(k) = A(k)\sinh(kb)\) and \(B^*(k) = B(k)\sinh(kb)\),
and let's multiple through by \(\cos(k'x)\).
\begin{alignat}{2}
\phi(x, 0) &= \int_{-\infty}^{\infty}\int_0^{\infty}
\left[A^*(k)\cos(kx)\cos(k'x) + B^*(k)\sin(kx)\cos(k'x)\right]dkdx
&& ={} \int_{-\infty}^{\infty}f(x)\cos(k'x)dx\\
\phi(x, 0) &= \int_0^{\infty}\int_{-\infty}^{\infty}
A^*(k)\cos(kx)\cos(k'x)dxdk && ={} \int_{-\infty}^{\infty}f(x)\cos(k'x)dx
\end{alignat}
Note that \(\cos(kx)\cos(k'x) = \frac{1}{2}\left[\cos(x\Delta k) +
\cos[x(k + k')]\right]\).
From class, we know that the integral of \(\cos[x(k + k')]\) is zero.
\begin{align}
\int_{-\infty}^{\infty}\cos(kx)\cos(k'x)dx &=
\int_{0}^{\infty}\cos(x\Delta k)dx\\
&= \mathcal{R}e
\left\{\int_0^{\infty}\exp\left[(i\Delta k - \eta)x\right]dx\right\}\\
&= \mathcal{R}e\left\{\left.
\frac{\exp\left[(i\Delta k - \eta)x\right]}
{i\Delta k - \eta}\right|_0^{\infty}\right\}\\
&= \mathcal{R}e\left\{\frac{1}{-i\Delta k + \eta}\right\}\\
&= \frac{\eta}{\eta^2 + (\Delta k)^2}
\end{align}
In class, we have shown that
\(\frac{\eta}{\eta^2 + (\Delta k)^2} = \pi\delta(k - k')\).
Therefore, we have that
\[
\int_{-\infty}^{\infty}\cos(kx)\cos(k'x)dx = \pi\delta(k - k').
\]
Now we have that
\[
\phi(x, 0) = \pi\int_0^{\infty}A^*(k)\delta(k - k')dk = \int_{-\infty}^{\infty}f(x)\cos(k'x)dx
\]
The integral \(\int_0^{\infty}A^*(k)\delta(k - k')dk = A^*(k')\) so
\begin{align}
A^*(k') &= \frac{1}{\pi}\int_{-\infty}^{\infty}f(x)\cos(k'x)dx\\
A(k) &= \frac{1}{\pi\sinh(kb)}\int_{-\infty}^{\infty}f(x')\cos(kx')dx'
\end{align}
Now we can write the general solution \cref{lapgensoln} as
\[
\phi(x, y) = \frac{1}{\pi}\int_0^{\infty}\int_{-\infty}^{\infty}
\frac{\sinh(k(b - y))}{\sinh(kb)}f(x')\cos(kx)\cos(kx')dx'dk.
\]
Again we have that \(\cos(kx)\cos(kx') = \frac{1}{2}\left[\cos(k(x' - x)) +
\cos(k(x' + x))\right]\) and \(\cos(k(x' + x))\) integrates out to zero.
Let \(x' = \xi\) and \(k = u\).
Then the solution is
\[
\phi(x, y) = \frac{1}{2\pi}\int_0^{\infty}\int_{-\infty}^{\infty}
\frac{\sinh(u(b - y))}{\sinh(ub)}f(\xi)\cos(u(\xi - x))d\xi dk.
\]
I have an extra factor of 1/2. Where am I missing a factor of 2?
 
Last edited:

Similar threads

Replies
5
Views
3K
Replies
17
Views
3K
Replies
4
Views
2K
Replies
5
Views
3K
Replies
3
Views
2K
Replies
5
Views
2K
Back
Top