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But if it is indefinite, shouldn't the factorization algorithm complain about a zero pivot?
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<div>Vincent<br>
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<div id="divRpF828991" style="direction: ltr;"><font face="Tahoma" size="2" color="#000000"><b>Von:</b> Matthew Knepley [knepley@gmail.com]<br>
<b>Gesendet:</b> Montag, 13. Oktober 2014 17:23<br>
<b>An:</b> De Groof, Vincent Frans Maria<br>
<b>Cc:</b> petsc-users@mcs.anl.gov<br>
<b>Betreff:</b> Re: [petsc-users] Convergence problem in nonlinear problem, with PCBDDC.<br>
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<div class="gmail_quote">On Mon, Oct 13, 2014 at 10:18 AM, De Groof, Vincent Frans Maria
<span dir="ltr"><<a href="mailto:Vincent.De-Groof@uibk.ac.at" target="_blank">Vincent.De-Groof@uibk.ac.at</a>></span> wrote:<br>
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<div style="direction:ltr; font-family:Tahoma; color:#000000; font-size:10pt">Hi,
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<div>I am having a convergence problem in a nonlinear FE problem (we only use ksp from petsc, not snes). We don't have any problems using a direct solver with a cholesky factorization (Petsc factor, cholmod, mumps). Now we want to solve it using the bddc preconditioner.
No problems on some problems. So far so good.</div>
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<div>Unfortunately, in some problems, the first increment in the newton iteration does not converge. It exits due to divergence (-4) after the first solve (0 iterations). This happens with CG, using the natural or preconditioned norm. Using the unpreconditioned
norm, or minres, the problem converges with no problems. Checking the Dirichlet and Neumann errors of the subdomains (using -pc_bddc_check_level 1) delivers small errors for all subdomains (~1e-16 - 1e-11).</div>
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<div>Because minres works, and cg doesn't, it seems like I am missing a null space. But since the subdomain solvers work without problems and do not complain
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<div>MINRES works for symmetric indefinite problems, whereas CG needs SPD problems. Could your problem be indefinite? If not, and it is close,</div>
<div>its possible that the preconditioned problem is indefinite.</div>
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<div> Matt</div>
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<div>about a null pivot this seems unlikely. Also when I check the number of constraints on the subdomains, they should be fully-determined...</div>
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<div>any suggestions on what might be going wrong here? Am I really missing a nullspace? Or might there be something else going wrong?</div>
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<div>kind regards,</div>
<div>Vincent</div>
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-- <br>
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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