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<div class="moz-cite-prefix">On 23/09/2024 17:16, Matthew Knepley
wrote:<br>
</div>
<blockquote type="cite" cite="mid:CAMYG4GmOv3UH1uQrNfH+E3x6aHAOvMNaN1AQR4kDW2j=NS1Kug@mail.gmail.com">
<div dir="ltr">
<div dir="ltr">On Mon, Sep 23, 2024 at 5:33 AM Matteo Semplice
via petsc-users <<a href="mailto:petsc-users@mcs.anl.gov" moz-do-not-send="true" class="moz-txt-link-freetext">petsc-users@mcs.anl.gov</a>>
wrote:<br>
</div>
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Dear
petsc,<br>
<br>
I need to hand-code a jacobian and I can't figure out
how to <br>
translate the DMplex points/fields to matrix indices.<br>
<br>
My DMPlex has a section with m fields per cell (which makes
for n>m dof <br>
per cell since some fields are vector). Say I want to insert
an nxn <br>
block for the row corresponding to cell c and coloumn of its
neighbour <br>
d. I guess that I should call either
MatSetValues/MatSetValuesLocal or <br>
the blocked variants, but how do I find the row/col indices
to pass in <br>
starting from the c/d dmplex points? And, while I am at it,
which <br>
MatSetValues version standard/Local standard/blocked is
best?<br>
<br>
I looked for a petsc example, but fails to find it: if
there's one, can <br>
you just point me to it?<br>
</blockquote>
<div><br>
</div>
<div>1. In FEM, the main unit of indices is usually the
closure, so we provide</div>
<div><br>
</div>
<div> <a href="https://urldefense.us/v3/__https://petsc.org/main/manualpages/DMPlex/DMPlexGetClosureIndices/__;!!G_uCfscf7eWS!YwF8ebZtFAZdHXKPu98AWB7KPuScfxMu4dRoe8ZqVaPVoEO_nX_qJa-zS6pOWHnPUs2ME6GB7l_SlHBl8ypyOPOQXzbVSC9xXhd8tw$" moz-do-not-send="true" class="moz-txt-link-freetext">https://petsc.org/main/manualpages/DMPlex/DMPlexGetClosureIndices/</a></div>
<div><br>
</div>
<div>which is what is used inside of</div>
<div><br>
</div>
<div> <a href="https://urldefense.us/v3/__https://petsc.org/main/manualpages/DMPlex/DMPlexMatSetClosure/__;!!G_uCfscf7eWS!YwF8ebZtFAZdHXKPu98AWB7KPuScfxMu4dRoe8ZqVaPVoEO_nX_qJa-zS6pOWHnPUs2ME6GB7l_SlHBl8ypyOPOQXzbVSC8i-x5AVA$" moz-do-not-send="true" class="moz-txt-link-freetext">https://petsc.org/main/manualpages/DMPlex/DMPlexMatSetClosure/</a></div>
<div><br>
</div>
<div>2. These indices are calculated using the global section,
so you can just</div>
<div><br>
</div>
<div> DMGetGlobalSection(dm, &gs);</div>
<div><br>
</div>
<div>and then use PetscSectionGetDof() and
PetscSectionGetOffset(), knowing that</div>
<div>off-process values are encoded as -(off + 1), so you need
to convert those.</div>
<div><br>
</div>
<div>Does this make sense?</div>
</div>
</div>
</blockquote>
<p>Yes! Thank you!</p>
<p>Matteo<br>
</p>
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