@article {Nielsen.2004.1,
	title = {Preconditioning by Inverting the Laplacian; an Analysis of the Eigenvalues},
	journal = {IMA Journal of Numerical Analysis},
	volume = {29},
	number = {1},
	year = {2009},
	month = {January},
	pages = {24-42},
	author = {Nielsen, Bj{\o}rn Fredrik and Tveito, Aslak and Hackbusch, Wolfgang}
}
