Analytical solution of the Green’s functions for the Poisson problem outside a disk

The Poisson problem in the space outside of a disk is investigated. Specifically, the Green’s function corresponding to the problem is calculated. The Green’s function equation is separable in terms of (oblate) ellipsoidal coordinates. Therefore, these coordinates are used to obtain a series expression for the Green’s function. This is done for both the Dirichlet and Neumann problems. The result is used to calculate the electric potential for some examples, with specific boundary conditions on a disk.