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<div id="version" align=right><b>petsc-3.4.2 2013-07-02</b></div>
<A NAME="MatGetSchurComplement"><H1>MatGetSchurComplement</H1></A>
Obtain the Schur complement from eliminating part of the matrix in another part.
<H3><FONT COLOR="#CC3333">Synopsis</FONT></H3>
<PRE>
#include "petscksp.h"
PetscErrorCode MatGetSchurComplement(Mat mat,IS isrow0,IS iscol0,IS isrow1,IS iscol1,MatReuse mreuse,Mat *newmat,MatReuse preuse,Mat *newpmat)
</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>mat </B></TD><TD>- Matrix in which the complement is to be taken
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>isrow0 </B></TD><TD>- rows to eliminate
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>iscol0 </B></TD><TD>- columns to eliminate, (isrow0,iscol0) should be square and nonsingular
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>isrow1 </B></TD><TD>- rows in which the Schur complement is formed
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>iscol1 </B></TD><TD>- columns in which the Schur complement is formed
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>mreuse </B></TD><TD>- MAT_INITIAL_MATRIX or MAT_REUSE_MATRIX, use MAT_IGNORE_MATRIX to put nothing in newmat
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>preuse </B></TD><TD>- MAT_INITIAL_MATRIX or MAT_REUSE_MATRIX, use MAT_IGNORE_MATRIX to put nothing in newpmat
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Output Parameters</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>newmat </B></TD><TD>- exact Schur complement, often of type MATSCHURCOMPLEMENT which is difficult to use for preconditioning
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>newpmat </B></TD><TD>- approximate Schur complement suitable for preconditioning
</TD></TR></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 diagonal of the (0,0) block
which will rarely produce a scalable algorithm.
<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
"MatNestGetSubMat_C" to their function. If their function needs to fall back to the default implementation, it
should call MatGetSchurComplement_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/MatCreateSchurComplement.html#MatCreateSchurComplement">MatCreateSchurComplement</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#MatGetSchurComplement">src/ksp/ksp/utils/schurm.c</A>
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