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Class notes for 2006/02/16

### Block systems and solvers

In many cases, it is advantageous to make use of structural information
about the system matrix. For example, for the mixed Laplace we
discussed last time, we know something about the block structure of the
system matrix. The
step-20 tutorial shows how to make use of this information, how to extract certain blocks of a matrix, and how to construct a Schur complement solver from this.