J. K. Kraus C. W. Brand Condition Numbers of Approximate Schur Complements in Two- and Three-Dimensional Discretizations on Hierarchically Ordered Grids. 135-154 2000 65 Computing 2 http://link.springer.de/link/service/journals/00607/bibs/0065002/00650135.htm db/journals/computing/computing65.html#KrausB00