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<p>OK, thank you Matt.</p>
<p>Manuel</p>
<p>---<br>
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<div class="moz-cite-prefix">On 3/25/19 6:27 PM, Matthew Knepley
wrote:<br>
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<div dir="ltr">On Mon, Mar 25, 2019 at 8:07 AM Manuel Colera
Rico via petsc-users <<a
href="mailto:petsc-users@mcs.anl.gov" moz-do-not-send="true">petsc-users@mcs.anl.gov</a>>
wrote:<br>
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<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">Hello,<br>
<br>
I would like to solve a N*N block system (with N>2) in
which some of the <br>
diagonal blocks are null. My system matrix is defined as a
MatNest. As <br>
N>2, I can't use "pc_fieldsplit_type schur" nor <br>
"pc_fieldsplit_detect_saddle_point". The other algorithms
("additive", <br>
"multiplicative" and "symmetric_multiplicative") don't work
either as <br>
they need each A_ii to be non-zero.<br>
<br>
Is there any built-in function in PETSc for this? If not,
could you <br>
please suggest a workaround?<br>
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<div>You can just shove all of the rows with nonzero diagonal
in one field, and all with zero diagonal in another, and do
Schur. This is what<br>
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<div> -pc_fieldsplit_detect_saddle_point</div>
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<div>does. However, you have to understand the Schur
complement to solve it efficiently. More generally, you can
recursively split the matrix,</div>
<div>which is what I do for many multiphysics problems.</div>
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<div> Thanks,</div>
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<div> Matt</div>
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Thanks and kind regards,<br>
<br>
Manuel<br>
<br>
---<br>
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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>
<div><br>
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<div><a href="http://www.cse.buffalo.edu/~knepley/"
target="_blank" moz-do-not-send="true">https://www.cse.buffalo.edu/~knepley/</a><br>
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