On Tue, Jul 21, 2009 at 3:16 PM, Umut Tabak <span dir="ltr"><<a href="mailto:u.tabak@tudelft.nl">u.tabak@tudelft.nl</a>></span> wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Dear all,<br>
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As a fresh user of Petsc libraries, should thank the developers for such a magnificent endeavor and years of work.<br>
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So the question directly related to Petsc is that if I have a singular system matrix and try to solve for the unknowns(simple enough 3 by 3) (I am using the simple linear system example from the Petsc user manual as a template where a preconditioner is used, I guess it is Jacobi.), I do not get any warnings for zero pivots in LU decomposition which I could not understand why, and the results are on the order of e+16, also the norm of the error. But why is not there some kind of warning.</blockquote>
<div><br>If your system is badly scaled, roundoff errors could result in a pivot larger than our tolerance. It is also possible that your preconditioner<br>resulted in a badly scaled system.<br><br> Matt<br> </div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
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The second part of the question is related to Slepc, this might not find direct answers here perhaps, but let me give it a try.<br>
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I have a generalized eigenvalue problem, it is a vibration related problem so I will use K and M instead of A and B, respectively. On my problem, K is singular, and if I use slepc to find the solution, petsc warns me about the zero pivot emergence, and breaks down naturally, there after I apply some shift operations that are already implemented in slepc to overcome the problem.<br>
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The question is what is the effect of preconditioner on a singular matrix for the linear system explained above, somehow, I was thinking in any case that should also warn me but it did not and gave some wrong results.<br>
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I am a bit weak on the preconditioners, maybe should have done some reading but I know that singular systems can also have solutions by some order tricks, pseudo inverse, temporary links application solutions with respect to rigid body modes(from structural mechanics too specific maybe).<br>
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Can Petsc handle singular systems as well? I am a bit confused at this point.<br>
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Best regards,<br><font color="#888888">
<br>
Umut<br>
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</font></blockquote></div><br><br clear="all"><br>-- <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<br>