On Wed, Feb 16, 2011 at 6:11 PM, Juha Jäykkä <span dir="ltr"><<a href="mailto:juhaj@iki.fi">juhaj@iki.fi</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;">
Hi again.<br>
<br>
I do not know which one to answer to, but thanks to all. I got one step<br>
further with your help:<br>
<br>
[0]PETSC ERROR: Zero pivot row 1 value 1.90611e-13 tolerance 1e-12!<br></blockquote><div><br></div><div>You have a singular Jacobian, which leads me to believe that your boundary</div><div>conditions are incorrect.</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;">
Adding -pc_factor_shift_type POSITIVE_DEFINITE helps and now I get at least<br>
some progress. It does not matter if I use -snes_fd, coloring or my hand-<br>
written Jacobian. If I start from a non-solution initial guess, they all<br>
progress somewhat towards what I believe is more or less a real solution. This<br>
is all ran with -pc_type lu -ksp_type preonly, btw.<br>
<br>
However, they do not seem to change the second to the last (counting from left<br>
to right) value on the lattice (this is 1D). It does change, but only by 0.02%<br>
while values elsewhere in the lattice change much more significantly. Again, I<br>
am running with the parameters where I know the exact solution and my initial<br>
guess is, on purpose, 0.01 off the correct one (except at the endpoints). What<br>
could be causing this?<br>
<br>
One thing strikes me as odd. I tried checking the value of the function at the<br>
first iteration and at that particular point it is of the order of -1.e-9,<br>
whereas in the next point to the left, it is +1.e-9 and then starts increasing<br>
towards the left before it starts decreasing again around half-way through the<br>
lattice.<br>
<br>
Cheers,<br>
Juha<br>
<font color="#888888"><br>
--<br>
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| 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>
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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>