Preconditioners for spectral discretizations of Helmholtz's equation with Sommerfeld boundary conditions

Some preconditioners for the iterative solution of Helmholtz's equation discretized with spectral Legendre collocation methods are introduced and studied. The preconditioners are based either on a finite element discretization of Helmholtz's equation on the spectral collocation mesh or on replacing the Sommerfeld-like boundary condition on a subset of the boundary with either Neumann or Dirichlet boundary conditions. The convergence rate of the resulting iterative methods is only mildly dependent on the spectral degree N and the wave number k