<div dir="ltr"><span style="font-size:12.8px">Especially in eigenproblems, I would suggest using Matthews approach, or applying Dirichlet boundary conditions via the penalty method (penalized robin for example). This would avoid spurious eigenvalues which occur when you use MatZeroRows().</span></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Oct 7, 2015 at 2:17 PM, Matthew Knepley <span dir="ltr"><<a href="mailto:knepley@gmail.com" target="_blank">knepley@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><span class="">On Wed, Oct 7, 2015 at 7:08 AM, Giuntoli Guido Alfredo <span dir="ltr"><<a href="mailto:ggiuntoli@tecna.com" target="_blank">ggiuntoli@tecna.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span lang="EN-US">Hi! I have a problem of the form Ax=kBx were x is a scalar variable that is 0 in the border of a domain (x_D=0).<u></u><u></u></span></p>
<p class="MsoNormal"><span lang="EN-US">Using FEM the problem that you have to solve is a reduced problem were you don’t have to include the coefficients of the matrix that multiply the x_Ds. So, I have two alternatives here, one is to construct a reduce matrix
A and B from the beginning… this could result in a slow code. The other alternative I want to try is to use a sub matrix of A and B and pass them to slepc. How can I do this is I have Mat A and Mat b ?</span></p></div></div></blockquote><div><br></div></span><div>1) Can't you just use MatZeroRows() or MatZeroRowsColumns() to apply your BC?</div><div><br></div><div>2) I am not sure why you think eliminating variables will be slower. My experiments and theoretical understanding say it is at least as fast as zeroing the rows.</div><div><br></div><div>3) If you want to extract a submatrix, you can use MatGetSubmatrix().</div><div><br></div><div> Thanks,</div><div><br></div><div> Matt</div><span class=""><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div lang="ES-AR" link="blue" vlink="purple"><div><p class="MsoNormal"> </p>
<p class="MsoNormal"><span lang="EN-US">Thank you, Guido!<u></u><u></u></span></p>
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