PDE25

From Exampleproblems

$\Delta u = u_{xx} + u_{yy}=0\,$

$u(x,0) = \begin{cases}T_0 & |x|<b \\ 0 & |x|>b\end{cases}\,$
$\lim_{x^2+y^2\rightarrow\infty} u(x,y) = 0\,$

$-\infty<x<\infty,\,\,\,\,\, 0<y<\infty\,$

Let $u = XY\,$ so that

$X''+\lambda^2X = 0,\,\,-\infty<x<\infty\,$

$Y''-\lambda^2Y = 0,\,\,0<y<\infty\,$

$X(x) = \begin{cases}\frac{1}{2}A_0 & \lambda=0 \\ c_1\cos \lambda x + c_2 \sin \lambda x & \lambda>0\end{cases}\,$

$Y(y) = \begin{cases}\frac{1}{2}B_0 & \lambda=0 \\ c_3 e^{\lambda x} + c_4 e^{-\lambda x} & \lambda>0\end{cases}\,$

The exponentials were chosen over the hyperbolic trig functions because one of the exponetials tends to zero as its argument tends to infinity.

To satisfy the limit condition, we must set $c_3 = A_0 = B_0 = 0\,$

The solution is

$u(x,y) = \int_0^\infty\left[A_\lambda\cos\lambda x + B_\lambda\sin\lambda x\right] e^{-\lambda x}\,d\lambda\,$

$u(x,0) = \int_0^\infty A_\lambda\cos \lambda x + B_\lambda \sin \lambda x = \begin{cases}T_0 & |x|<b \\ 0 & |x|>b\end{cases}\,$

The Fourier coefficients are:

$A_\lambda = \frac{1}{\pi} \int_{-b}^b T_0 \cos \lambda x\,dx = \frac{T_0}{\pi\lambda}\sin\lambda x\Bigg|_{-b}^b = \frac{2T_0\sin\lambda b}{\pi\lambda}\,$