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<div id="version" align=right><b>petsc-3.7.5 2017-01-01</b></div>
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<A NAME="MatSchurComplementGetPmat"><H1>MatSchurComplementGetPmat</H1></A>
Obtain a preconditioning matrix for the Schur complement by assembling Sp = A11 - A10 inv(diag(A00)) A01
<H3><FONT COLOR="#CC3333">Synopsis</FONT></H3>
<PRE>
#include "petscksp.h"
PetscErrorCode MatSchurComplementGetPmat(Mat S,MatReuse preuse,Mat *Sp)
</PRE>
Collective on <A HREF="../Mat/Mat.html#Mat">Mat</A>
<P>
<H3><FONT COLOR="#CC3333">Input Parameters</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>S </B></TD><TD>- matrix obtained with <A HREF="../KSP/MatCreateSchurComplement.html#MatCreateSchurComplement">MatCreateSchurComplement</A>() (or equivalent) and implementing the action of A11 - A10 ksp(A00,Ap00) A01
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>preuse </B></TD><TD>- MAT_INITIAL_MATRIX for a new Sp, or MAT_REUSE_MATRIX to reuse an existing Sp, or MAT_IGNORE_MATRIX to put nothing in Sp
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Output Parameter</FONT></H3>
<DT><B>Sp </B> -approximate Schur complement suitable for preconditioning S = A11 - A10 inv(diag(A00)) A01
<br>
</TABLE>
<P>
<H3><FONT COLOR="#CC3333">Note</FONT></H3>
Since the real Schur complement is usually dense, providing a good approximation to newpmat usually requires
application-specific information. The default for assembled matrices is to use the inverse of the diagonal of
the (0,0) block A00 in place of A00^{-1}. This rarely produce a scalable algorithm. Optionally, A00 can be lumped
before forming inv(diag(A00)).
<P>
Sometimes users would like to provide problem-specific data in the Schur complement, usually only
for special row and column index sets. In that case, the user should call <A HREF="../Sys/PetscObjectComposeFunction.html#PetscObjectComposeFunction">PetscObjectComposeFunction</A>() to set
"MatSchurComplementGetPmat_C" to their function. If their function needs to fall back to the default implementation,
it should call MatSchurComplementGetPmat_Basic().
<P>
<P>
<P>
<H3><FONT COLOR="#CC3333">See Also</FONT></H3>
<A HREF="../Mat/MatGetSubMatrix.html#MatGetSubMatrix">MatGetSubMatrix</A>(), <A HREF="../PC/PCFIELDSPLIT.html#PCFIELDSPLIT">PCFIELDSPLIT</A>, <A HREF="../KSP/MatGetSchurComplement.html#MatGetSchurComplement">MatGetSchurComplement</A>(), <A HREF="../KSP/MatCreateSchurComplement.html#MatCreateSchurComplement">MatCreateSchurComplement</A>(), <A HREF="../KSP/MatSchurComplementSetAinvType.html#MatSchurComplementSetAinvType">MatSchurComplementSetAinvType</A>()
<BR><P><B><P><B><FONT COLOR="#CC3333">Level:</FONT></B>advanced
<BR><FONT COLOR="#CC3333">Location:</FONT></B><A HREF="../../../src/ksp/ksp/utils/schurm.c.html#MatSchurComplementGetPmat">src/ksp/ksp/utils/schurm.c</A>
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