Jed Brown wrote:

On Tue, Mar 29, 2011 at 12:55, Thomas Witkowski <[email protected] <mailto:[email protected]>> wrote:

I have implemented a Schur complement solver based on the fieldsplit preconditioner, i.e., my code is executed with the option -pc_fieldsplit_type schur. How can I define a preconditioner Sp for the inner Schur complement solution? I want to define the preconditioner in an explicit way, thus build a matrix that should approximate S in some way!

See PCFieldSplitSchurPrecondition and (perhaps) PCShell. Perhaps if you give a more specific example, we can give more specific suggestions. PCFieldsplitSchurPrecondition seems to do what I want. But how to create the matrix for the preconditioner? I'm a little bit confused about it because the matrix/operator S acts on an index range that does not start from 0 and is not continuous, because each rank owns only some parts of the interior domain boundaries.

Thomas