@article {GreifSchotzau2006, AUTHOR = {Greif, Chen and Sch{\"o}tzau, Dominik}, TITLE = {Preconditioners for saddle point linear systems with highly singular {$(1,1)$} blocks}, JOURNAL = {Electron. Trans. Numer. Anal.}, VOLUME = {22}, YEAR = {2006}, PAGES = {114--121} }