### 3.2 Laplace PDE outside disk

Problem Solve $$u_{xx}+u_{yy}=0$$ outside disk $$x^{2}+y^{2}>1$$ with boundary condition $$xy^{2}$$ when $$x^{2}+y^{2}=a$$. Where $$a=1$$ in this problem.

The ﬁrst step is to convert the boundary condition to polar coordinates. Since $$x=r\cos \theta ,y=r\sin \theta$$, then at the boundary $$u\left ( r,\theta \right ) =r\cos \theta \left ( r\sin \theta \right ) ^{2}$$. But $$r=1$$ (the radius). Hence at the boundary, $$u\left ( 1,\theta \right ) =f\left ( \theta \right )$$ where \begin{align*} f\left ( \theta \right ) & =\cos \theta \sin ^{2}\theta \\ & =\cos \theta \left ( 1-\cos ^{2}\theta \right ) \\ & =\cos \theta -\cos ^{3}\theta \end{align*}

But $$\cos ^{3}\theta =\frac{3}{4}\cos \theta +\frac{1}{4}\cos 3\theta$$. Therefore the above becomes\begin{align} f\left ( \theta \right ) & =\cos \theta -\left ( \frac{3}{4}\cos \theta +\frac{1}{4}\cos 3\theta \right ) \nonumber \\ & =\frac{1}{4}\cos \theta -\frac{1}{4}\cos 3\theta \tag{1} \end{align}

The above is also seen as the Fourier series of $$f\left ( \theta \right )$$. The PDE in polar coordinates is$u_{rr}+\frac{1}{r}u_{r}+\frac{1}{r^{2}}u_{\theta \theta }=0$ The solution is known to be$$u\left ( r,\theta \right ) =\frac{c_{0}}{2}+\sum _{n=1}^{\infty }r^{-n}\left ( c_{n}\cos \left ( n\theta \right ) +k_{n}\sin \left ( n\theta \right ) \right ) \tag{2}$$ Since the above solution is the same as $$f\left ( \theta \right )$$ when $$r=1$$, then equating (2) when $$r=1$$ to (1) gives$\frac{1}{4}\cos \theta -\frac{1}{4}\cos 3\theta =\frac{c_{0}}{2}+\sum _{n=1}^{\infty }\left ( c_{n}\cos \left ( n\theta \right ) +k_{n}\sin \left ( n\theta \right ) \right )$ By comparing terms on both sides, this shows by inspection that\begin{align*} c_{0} & =0\\ c_{1} & =\frac{1}{4}\\ c_{3} & =\frac{-1}{4} \end{align*}

And all other $$c_{n},k_{n}$$ are zero. Using the above result back in (2) gives the solution as $$u\left ( r,\theta \right ) =\frac{r^{-1}}{4}\cos \theta -\frac{r^{-3}}{4}\cos 3\theta \tag{3}$$ This is 3D plot of the solution

This is a contour plot