<html><body><div style="color:#000; background-color:#fff; font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:12pt"><div><span>Matt,</span></div><div><span></span> </div><div><span>The linear system is hard to solve and may be ill-conditioned. I got the condition number from PETSc, which does not look too bad (around 10^4), then the distribution of eigenvalues (spectrum) will be helpful for choosing suitable reordering/preconditioning types.</span></div><div><span></span> </div><div><span>Thanks,</span></div><div><span>Qin</span></div><div><br></div> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 12pt;"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 12pt;"> <div dir="ltr"> <div style="margin: 5px 0px; padding: 0px; border: 1px solid rgb(204, 204, 204); height:
0px; line-height: 0; font-size: 0px;" class="hr" contentEditable="false" readonly="true"></div> <font size="2" face="Arial"> <b><span style="font-weight: bold;">From:</span></b> Matthew Knepley <knepley@gmail.com><br> <b><span style="font-weight: bold;">To:</span></b> Qin Lu <lu_qin_2000@yahoo.com> <br><b><span style="font-weight: bold;">Cc:</span></b> "petsc-users@mcs.anl.gov" <petsc-users@mcs.anl.gov> <br> <b><span style="font-weight: bold;">Sent:</span></b> Wednesday, February 26, 2014 4:46 PM<br> <b><span style="font-weight: bold;">Subject:</span></b> Re: [petsc-users] Eigenvalues calculations in PETSc<br> </font> </div> <div class="y_msg_container"><br><div id="yiv0952974797"><div><div dir="ltr"><div class="yiv0952974797gmail_extra"><div class="yiv0952974797gmail_quote">On Wed, Feb 26, 2014 at 2:41 PM, Qin Lu <span dir="ltr"><<a href="mailto:lu_qin_2000@yahoo.com" shape="rect" rel="nofollow" target="_blank"
ymailto="mailto:lu_qin_2000@yahoo.com">lu_qin_2000@yahoo.com</a>></span> wrote:<br clear="none">
<blockquote style="margin: 0px 0px 0px 0.8ex; padding-left: 1ex; border-left-color: rgb(204, 204, 204); border-left-width: 1px; border-left-style: solid;" class="yiv0952974797gmail_quote">Hello,<br clear="none">
<br clear="none">
I need to find the spectrum of the a large linear system (over 100,000 unknowns). Option -ksp_compute_eigenvalues_explicitly ran out of memory since the matrix is too big, then -ksp_compute_eigenvalues only gave a small number of eigenvalues (around 30). My questions are:<br clear="none">
<br clear="none">
1. Is the number of computed eigenvalues determined by the number of GMRES iterations to converge?<br clear="none">
2. Can PETSc give the extreme eigenvalues now that the number of computed eigenvalues is limited?<br clear="none"></blockquote><div><br clear="none"></div><div>It will help to understand why you want these.</div><div><br clear="none"></div><div> Matt</div><div id="yiv0952974797yqtfd07001" class="yiv0952974797yqt6940090598"><div>
</div></div><blockquote style="margin: 0px 0px 0px 0.8ex; padding-left: 1ex; border-left-color: rgb(204, 204, 204); border-left-width: 1px; border-left-style: solid;" class="yiv0952974797gmail_quote"><div id="yiv0952974797yqtfd71561" class="yiv0952974797yqt6940090598">
Thanks a lot for your info,<br clear="none">
Qin </div><br clear="none">
</blockquote></div><br clear="none"><br clear="all"><div><br clear="none"></div>-- <br clear="none">What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br clear="none">
-- Norbert Wiener
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