@article {14152,
	title = {A multigrid method enhanced by Krylov subspace iteration for discrete Helmholtz equations},
	journal = {SIAM Journal on scientific computing},
	volume = {23},
	year = {2002},
	month = {2002///},
	pages = {1291 - 1315},
	author = {Elman, Howard and Ernst, O. G and O{\textquoteright}Leary, Dianne P. and others}
}
