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<p style="margin-top:0;margin-bottom:0">Hello, </p>
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<p style="margin-top:0;margin-bottom:0">I am trying to use PETSC to solve a singular system like the following.</p>
<p style="margin-top:0;margin-bottom:0"><img size="3538" contenttype="image/png" id="img266709" tabindex="0" style="max-width: 99.9%; user-select: none;" data-outlook-trace="F:1|T:1" src="cid:ef7b4672-623e-48d7-b9fa-6e8d4ed67b5f">.<br>
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<p style="margin-top:0;margin-bottom:0">This equals inserting rows and columns at arbitrary place(Actually I even don't know whether it is singular or not. Sometimes some degree of freedom x_i may not participate in the global matrix)</p>
<p style="margin-top:0;margin-bottom:0"><img size="2573" contenttype="image/png" id="img980344" tabindex="0" style="max-width: 99.9%; user-select: none;" data-outlook-trace="F:1|T:1" src="cid:0c347c3e-462d-4fe8-9718-273339c2c1f2"><br>
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<p style="margin-top:0;margin-bottom:0">I searched some discussion online and tried to remove the nullspace with the following code:</p>
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<div> call KSPCreate(PETSC_COMM_WORLD,ksp,ierr)</div>
<div> call KSPSetOperators(ksp,Amat,Amat,ierr)</div>
<div> call KSPSetFromOptions(ksp,ierr)</div>
<div> call KSPSetType(ksp, KSPCG,ierr) </div>
<div> call MatNullSpaceCreate( PETSC_COMM_WORLD, PETSC_TRUE, 0, dummyVecs, nullspace, ierr)</div>
<div> call MatSetNullSpace(Amat,nullspace,ierr)</div>
<div> call MatNullSpaceDestroy(nullspace,ierr)</div>
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<div> call KSPSolve(ksp,bvec,xvec,ierr)</div>
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<div>But it still gives me some error saying <span style="font-variant-ligatures: no-common-ligatures; font-family: Menlo; font-size: 11px;">Zero pivot in LU factorization.</span></div>
<div><span style="font-family: Calibri, Helvetica, sans-serif, Helvetica, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;">I am just wondering how MatSetNullSpace() can handle
this?</span><span style="font-size: 16px; background-color: rgb(213, 234, 255);"><font face="Times"></font></span></div>
<div><span style="font-family: Calibri, Helvetica, sans-serif, Helvetica, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;"><br>
</span></div>
<div><span style="font-family: Calibri, Helvetica, sans-serif, Helvetica, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;">Thanks</span></div>
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<p><span style="color:rgb(33,33,33); font-size:13.3333330154419px">Yaxiong Chen, </span><br style="color:rgb(33,33,33); font-size:13.3333330154419px">
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<p style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:16px"><span style="font-size:13.3333px">School of Mechanical Engineering, 3171 </span></p>
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<p style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:16px"><span style="font-size:13.3333px">West Lafayette, IN 47907</span></p>
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