Directional Preconditioner for High Frequency Obstacle Scattering
  The boundary integral method is an efficient approach for solving time-harmonic obstacle scattering problems by a bounded scatterer. This paper presents the directional preconditioner for the iterative solution of linear systems of the boundary integral method. This new preconditioner builds a data-sparse approximation of the integral operator, transforms it into a sparse linear system, and computes an approximate inverse with efficient sparse and hierarchical linear algebra algorithms. This preconditioner is efficient and results in small and almost frequency-independent iteration counts when combined with standard iterative solvers. Numerical results are provided to demonstrate the effectiveness of the new preconditioner. 
Lexing Ying
math.NA