@article{etna_vol31_pp403-424,
  author  = {Yogi A. Erlangga and Reinhard Nabben},
  title   = {On a multilevel Krylov method for the Helmholtz equation preconditioned by shifted Laplacian},
  journal = {Electron. Trans. Numer. Anal.},
  volume  = {31},
  year    = {2008},
  pages   = {403--424},
}