Ivan Georgiev Johannes Kraus Svetozar Margenov Multilevel Preconditioning of 2D Rannacher-Turek FE Problems; Additive and Multiplicative Methods. 56-64 2006 conf/nma/2006 Numerical Methods and Applications http://dx.doi.org/10.1007/978-3-540-70942-8_6 db/conf/nma/nma2006.html#GeorgievKM06