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<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US">Thank you Barry. This is great!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US">I plan to solve using ‘-</span><span style="font-size:11.0pt">pc_type redistribute’ after applying the Dirichlet bc using
<o:p></o:p></span></p>
<p class="p1"><span class="s1">MatZeroRowsColumnsIS</span>(<span class="s2">A</span>,
<span class="s2">isout</span>, <span class="s3">1</span>, <span class="s2">x</span>,
<span class="s2">b)</span>; <o:p></o:p></p>
<p class="p1"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">While I retrieve the solution data from x (after the solve) – can I index them using the original ordering (if I may say that)?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Kind regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Karthik.</span><span style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><b><span style="font-size:12.0pt;color:black">From:
</span></b><span style="font-size:12.0pt;color:black">Barry Smith <bsmith@petsc.dev><br>
<b>Date: </b>Tuesday, 10 January 2023 at 16:04<br>
<b>To: </b>Chockalingam, Karthikeyan (STFC,DL,HC) <karthikeyan.chockalingam@stfc.ac.uk><br>
<b>Cc: </b>petsc-users@mcs.anl.gov <petsc-users@mcs.anl.gov><br>
<b>Subject: </b>Re: [petsc-users] Eliminating rows and columns which are zeros<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><a href="https://petsc.org/release/docs/manualpages/PC/PCREDISTRIBUTE/#pcredistribute">https://petsc.org/release/docs/manualpages/PC/PCREDISTRIBUTE/#pcredistribute</a> -pc_type redistribute<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">It does everything for you. Note that if the right hand side for any of the "zero" rows is nonzero then the system is inconsistent and the system does not have a solution.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Barry<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<br>
<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">On Jan 10, 2023, at 10:30 AM, Karthikeyan Chockalingam - STFC UKRI via petsc-users <petsc-users@mcs.anl.gov> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Hello,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">I am assembling a MATIJ of size N, where a very large number of rows (and corresponding columns), are zeros. I would like to potentially eliminate them before the solve.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">For instance say N=7<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:9.0pt;font-family:Menlo;color:#5D6C79">0 0<span class="apple-converted-space"> </span>0<span class="apple-converted-space"> </span>0 0 0 0<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:9.0pt;font-family:Menlo;color:#5D6C79">0 1 -1<span class="apple-converted-space"> </span>0 0 0 0<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:9.0pt;font-family:Menlo;color:#5D6C79">0 -1 2<span class="apple-converted-space"> </span>0 0 0 -1<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:9.0pt;font-family:Menlo;color:#5D6C79">0 0<span class="apple-converted-space"> </span>0<span class="apple-converted-space"> </span>0 0 0 0<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:9.0pt;font-family:Menlo;color:#5D6C79">0 0<span class="apple-converted-space"> </span>0<span class="apple-converted-space"> </span>0 0 0 0<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:9.0pt;font-family:Menlo;color:#5D6C79">0 0<span class="apple-converted-space"> </span>0<span class="apple-converted-space"> </span>0 0 0 0<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-size:9.0pt;font-family:Menlo;color:#5D6C79">0 0<span class="apple-converted-space"> </span>-1 0 0 0 1<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">I would like to reduce it to a 3x3<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">1 -1 0<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">-1 2 -1<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">0 -1 1<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">I do know the size N.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Q1) How do I do it?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Q2) Is it better to eliminate them as it would save a lot of memory?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Q3) At the moment, I don’t know which rows (and columns) have the zero entries but with some effort I probably can find them. Should I know which rows (and columns) I am eliminating?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Thank you.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Karthik.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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