Derive the general form of separation solution
$$
\phi(r, \theta)=A \theta+B \ln (r / a)+\sum_{n=0}^{\infty}\left(C_{n}
r^{n}+D_{n} r^{-n}\right) \cos \left(n \theta+\alpha_{n}\right)
$$
for the equation \(\nabla^{2} \phi=0\) and the region
$$
\left\\{\begin{array}{r}
0
