Spectral solvers for spherical elliptic problems

A new family of direct spectral solvers for the 3D Helmholtz equation in a spherical gap and inside a sphere for nonaxisymmetric problems is presented. A variational formulation (no collocation) is adopted, based on the Fourier expansion and the associated Legendre functions to represent the angular dependence over the sphere and using basis functions generated by Legendre or Jacobi polynomials to represent the radial structure of the solution. In the present method, boundary conditions on the polar axis and at the sphere center are not required and never mentioned, by construction. The spectral solution of the vector Dirichlet problem is also considered, by employing a transformation that uncouples the spherical components of the Fourier modes and that is implemented here for the first time. The condition numbers of the matrices involved in the scalar solvers are computed and the spectral convergence of all the proposed solution algorithms is verified by numerical tests.