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I will, as soon as possible...</div>
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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> Tuesday, December 7, 2021 7:25:43 PM<br>
<b>To:</b> Marco Cisternino <marco.cisternino@optimad.it><br>
<b>Cc:</b> petsc-users <petsc-users@mcs.anl.gov><br>
<b>Subject:</b> Re: [petsc-users] Nullspaces</font>
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<div dir="ltr">On Tue, Dec 7, 2021 at 11:19 AM Marco Cisternino <<a href="mailto:marco.cisternino@optimad.it">marco.cisternino@optimad.it</a>> wrote:<br>
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<p class="x_MsoNormal">Good morning,<u></u><u></u></p>
<p class="x_MsoNormal"><span lang="EN-GB">I’m still struggling with the Poisson equation with Neumann BCs.<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">I discretize the equation by finite volume method and I divide every line of the linear system by the volume of the cell. I could avoid this division, but I’m trying to understand.<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">My mesh is not uniform, i.e. cells have different volumes (it is an octree mesh).<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">Moreover, in my computational domain there are 2 separated sub-domains.<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">I build the null space and then I use MatNullSpaceTest to check it.<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB"><u></u> <u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">If I do this:<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">MatNullSpaceCreate(getCommunicator(), PETSC_TRUE, 0, nullptr, &nullspace);<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">It works</span></p>
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<div>This produces the normalized constant vector.</div>
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<p class="x_MsoNormal"><span lang="EN-GB">If I do this:<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="FR">Vec nsp;<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="FR">VecDuplicate(m_rhs, &nsp);<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="FR">VecSet(nsp,1.0);<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="FR">VecNormalize(nsp, nullptr);<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">MatNullSpaceCreate(getCommunicator(), PETSC_FALSE, 1, &nsp, &nullspace);<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">It does not work</span></p>
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<div>This is also the normalized constant vector.</div>
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<div>So you are saying that these two vectors give different results with MatNullSpaceTest()?</div>
<div>Something must be wrong in the code. Can you send a minimal example of this? I will go</div>
<div>through and debug it.</div>
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<div> Thanks,</div>
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<div> Matt</div>
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<p class="x_MsoNormal"><span lang="EN-GB">Probably, I have wrong expectations, but should not it be the same?<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB"><u></u> <u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">Thanks<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB"><u></u> <u></u></span></p>
<p class="x_MsoNormal"><span>Marco Cisternino, PhD<br>
<a href="mailto:marco.cisternino@optimad.it" target="_blank"><span style="color:blue">marco.cisternino@optimad.it</span></a><u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">______________________<u></u><u></u></span></p>
<p class="x_MsoNormal"><span lang="EN-GB">Optimad Engineering Srl<u></u><u></u></span></p>
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<p class="x_MsoNormal"><span lang="EN-GB"><u></u> <u></u></span></p>
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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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