@article{ETNA_2008__31__a0,
     author = {Erlangga, Yogi A. and Nabben, Reinhard},
     title = {On a multilevel {Krylov} method for the {Helmholtz} equation preconditioned by shifted {Laplacian}},
     journal = {Electronic transactions on numerical analysis},
     pages = {403--424},
     publisher = {mathdoc},
     volume = {31},
     year = {2008},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ETNA_2008__31__a0/}
}