On Tue, Jul 21, 2009 at 4:09 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;">
<div class="im">On Tue, Jul 21, 2009 at 03:56:11PM -0500, Matthew Knepley wrote:<br>
> If your system is badly scaled, roundoff errors could result in a pivot<br>
> larger than our tolerance. It is also possible that your preconditioner<br>
> resulted in a badly scaled system.<br>
><br>
> Matt<br>
><br>
</div>Hi,<br>
Thanks for the fast reply.<br>
since the system is singular condition number will be bad any way,<br>
but I was wondering if there were already ways to overcome this<br>
problem. And are there ways to prevent the preconditioner to give a<br>
badly scaled system, what I mean is that the system is singular so<br>
will preconditioning improve that? I definitely read about these.<br></blockquote><div><br>Well, the default is ILU which can do horrendously things. Try running with<br>Jacobi as a test. It should fail like you want. In my opinion, blackbox PCs<br>
almost never work, and what you really need is something tailored to your<br>problem (which is linear algebra heresy).<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;">
<br>
Thanks and best,<br>
Umut<br>
<font color="#888888"><br>
--<br>
Quote: You can not teach a man anything, you can only help him find it within himself.<br>
Source: Galileo Galilei<br>
</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>