A parallel multigrid-based preconditioner for the 3D heterogeneous high-frequency Helmholtz equation. - C. D. Riyanti; A. Kononov; Yogi A. Erlangga; Cornelis Vuik; Cornelis W. Oosterlee; René-Edouard Plessix; Wim A. Mulder (J. Comput. Phys., 2007)