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<p>Hi Matt,</p>
<p><br>
</p>
<p>Thanks for your quick reply. </p>
<p>I have not completely understood your suggestion, could you please elaborate a bit more? </p>
<p>For your convenience, here is how I am proceeding for the moment in my code: </p>
<p><br>
</p>
<p>TSGetKSP(ts,&ksp);<br>
</p>
<p>KSPGetPC(ksp,&pc); <br>
</p>
<p>PCSetType(pc,PCFIELDSPLIT);</p>
<p>PCFieldSplitSetDetectSaddlePoint(pc,PETSC_TRUE);</p>
<p>PCSetUp(pc);</p>
<p>PCFieldSplitGetSubKSP(pc, &n, &subksp);</p>
<p>KSPGetPC(subksp[1], &(subpc[1]));</p>
<p><span style="font-size: 12pt;">KSPSetOperators(subksp[1],T,T);</span><br>
</p>
<p>KSPSetUp(subksp[1]);</p>
<p>PetscFree(subksp);</p>
<p>TSSolve(ts,X);<br>
</p>
<p><br>
</p>
<p>Thank you.</p>
<p>Best,</p>
<p><br>
</p>
<p>Zakariae</p>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Matthew Knepley <knepley@gmail.com><br>
<b>Sent:</b> Wednesday, July 7, 2021 12:11:10 PM<br>
<b>To:</b> Jorti, Zakariae<br>
<b>Cc:</b> petsc-users@mcs.anl.gov; Tang, Qi; Tang, Xianzhu<br>
<b>Subject:</b> [EXTERNAL] Re: [petsc-users] Problem with PCFIELDSPLIT</font>
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<div dir="ltr">On Wed, Jul 7, 2021 at 1:51 PM Jorti, Zakariae via petsc-users <<a href="mailto:petsc-users@mcs.anl.gov">petsc-users@mcs.anl.gov</a>> wrote:<br>
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<p>Hi,</p>
<p><br>
</p>
<p>I am trying to build a PCFIELDSPLIT preconditioner for a matrix </p>
<p>J = [A00 A01]</p>
<p> [A10 A11] </p>
<p>that has the following shape: </p>
<p><br>
</p>
<p>M_{user}^{-1} = [I -ksp(A00) A01] [ksp(A00) 0] [I 0]</p>
<p> [0 I] [0 ksp(T)] [-A10 ksp(A00) I ]<br>
</p>
<p><br>
</p>
<p>where T is a user-defined Schur complement approximation that replaces the true Schur complement S:= A11 - A10 ksp(A00) A01.</p>
<p><br>
</p>
<p>I am trying to do something similar to this example (lines 41--45 and 116--121): <a href="https://www.mcs.anl.gov/petsc/petsc-current/src/snes/tutorials/ex70.c.html" id="gmail-m_4230588803823514004LPlnk826214" target="_blank">https://www.mcs.anl.gov/petsc/petsc-current/src/snes/tutorials/ex70.c.html</a></p>
<p><br>
</p>
<p>The problem I have is that I manage to replace S with T on a separate single linear system but not for the linear systems generated by my time-dependent PDE. Even if I set the preconditioner <span style="font-size:12pt">M_{user}^{-1} correctly, the T matrix
gets replaced by S in the preconditioner once I call TSSolve. </span></p>
<p><span style="font-size:12pt">Do you have any suggestions how to fix this knowing that the matrix J does not change over time?</span></p>
<p><span style="font-size:12pt"></span></p>
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</blockquote>
<div>I don't like how it is done in that example for this very reason.</div>
<div><br>
</div>
<div>When I want to use a custom preconditioning matrix for the Schur complement, I always give a preconditioning matrix M to the outer solve.</div>
<div>Then PCFIELDSPLIT automatically pulls the correct block from M, (1,1) for the Schur complement, for that preconditioning matrix without</div>
<div>extra code. Can you do this?</div>
<div><br>
</div>
<div> Thanks,</div>
<div><br>
</div>
<div> Matt</div>
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<p><span style="font-size:12pt">Many thanks.</span></p>
<p><span style="font-size:12pt"><br>
</span></p>
<p><span style="font-size:12pt">Best regards,</span></p>
<p><span style="font-size:12pt"><br>
</span></p>
<p><span style="font-size:12pt">Zakariae </span></p>
<p><span style="font-size:12pt"></span></p>
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-- <br>
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<div>What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br>
-- Norbert Wiener</div>
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<div><a href="http://www.cse.buffalo.edu/~knepley/" target="_blank">https://www.cse.buffalo.edu/~knepley/</a><br>
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