An NG-based algorithm for a combined-field integral formulation

A fast and stable algorithm for solving external problems of scalar scattering is implemented. The method employs a combined-field formulation, which provides stable convergence of the iterative solver, and a multilevel nonuniform grid (NG) based algorithm for the fast evaluation of field integrals to overcome the O(N
²
) computational complexity of the direct computation. The method is used for calculating the scattered fields by fairly large scatterers, including non convex bodies with piece-wise smooth surfaces.