References

[1] Y.A. Erlangga and R. Nabben. On a multilevel Krylov method for the Helmholtz equation preconditioned by shifted Laplacian. Electronic Transactions on Numerical Analysis (ETNA), 31:403­424, 2008.
[2] Y.A. Erlangga, C.W. Oosterlee, and C. Vuik. A novel multigrid based preconditioner for heterogeneous Helmholtz problems. SIAM J. Sci. Comput, 27:1471­1492, 2006.
[3] J.M. Tang, S.P. MacLachlan, R. Nabben, and C. Vuik A Comparison of Two-Level Preconditioners Based on Multigrid and Deflation SIAM. J. Matrix Anal. and Appl., 31: 1715-1739, 2010