For a smooth domain \(D\subset \mathbb {R}^n\), solve Poisson’s equation \[ \nabla ^2u=f\text { in }D,\qquad \frac {\partial u}{\partial n}=g\text { on }\partial D \] in terms of an
appropriately defined Green’s function.
Show that the two-dimensional Green’s function for the Laplacian in an
unbounded domain is \[ G(x,x')=\frac {1}{2\pi }\ln \lvert x-x'\rvert . \]
Use the method of images to derive the Green’s function for the Laplacian
in the half-space \(x\in \mathbb {R}\), \(y>0\) with a Neumann boundary condition on \(y=0\), and evaluate
the corresponding solution of part (a) when \(f=0\).