On Wed, Feb 16, 2011 at 10:40 AM, Barry Smith <span dir="ltr"><<a href="mailto:bsmith@mcs.anl.gov">bsmith@mcs.anl.gov</a>></span> wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<br>
Try using SNESDefaultComputeJacobian() see if that makes any difference.<br>
<br>
99.9% of the causes of non-convergencing Newton are wrong or slightly wrong Jacobians. Very unlikely possibilities are<br>
<br>
1) it is converging to a local minimum that is not a solution. This is checked by PETSc automatically if the line search failed so is unlikely to be the problem. But run with -info and it will print a great deal of information about the nonlinear solver including a message about " near zero implies" cut and paste all the message about the "near zero" and send it to us.<br>
<br>
2) the function is not smooth so Newton's taylor series approximation simply doesn't work.</blockquote><div><br></div><div>Unlikely possibility #3:</div><div><br></div><div> You have written an equation with no real solutions, meaning there is a mistake in your function.</div>
<div><br></div><div> Matt</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><font color="#888888"><br>
Barry<br>
</font><div><div></div><div class="h5"><br>
On Feb 16, 2011, at 10:31 AM, Juha Jäykkä wrote:<br>
<br>
>> Yes, it is the most common place to make programming mistakes and the<br>
>> symptoms you describe are typical.<br>
><br>
> Please let me double-check there has not been a misunderstanding here: the<br>
> problems I describe occur with the PETSc built-in FD Jacobian approximation,<br>
> not my own. Now, I realise this will be a less-than-optimal approximation, but<br>
> I fail to see how there could be a programming mistake, when I am using<br>
> SNESDefaultComputeJacobianColor and not my hand-written Jacobian.<br>
><br>
> I do get the same symptoms with the hand-written one, too. That's why I wanted<br>
> to check with the PETSc built in FD version.<br>
><br>
> Cheers,<br>
> Juha<br>
><br>
> --<br>
> -----------------------------------------------<br>
> | Juha Jäykkä, <a href="mailto:juhaj@iki.fi">juhaj@iki.fi</a> |<br>
> | <a href="http://www.maths.leeds.ac.uk/~juhaj" target="_blank">http://www.maths.leeds.ac.uk/~juhaj</a> |<br>
> -----------------------------------------------<br>
<br>
</div></div></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>