Ralf Hiptmair Thomas Schiekofer Barbara I. Wohlmuth Multilevel Preconditioned Augmented Lagrangian Techniques for 2ndMixed Problems. 25-48 1996 57 Computing 1 http://link.springer.de/link/service/journals/00607/bibs/6057001/60570025.htm db/journals/computing/computing57.html#HiptmairSW96