On 5/3/06, <b class="gmail_sendername">Paul T. Bauman</b> <<a href="mailto:pbauman@ices.utexas.edu">pbauman@ices.utexas.edu</a>> wrote:<div><span class="gmail_quote"></span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Hello,<br><br>I'm a new PETSc user, so if the questions I ask are redundant, I<br>sincerely apologize. I was, however, unavailable to find the following<br>information in the manual pages.<br><br>I have two questions.<br>
<br>First: When using the SNES package, is the initial guess for the<br>/iterative linear solver/ (e.g. conjugate gradient) 0.0 or is it<br>something else (e.g. the current Newton solution)?</blockquote><div><br>
If you want that, you need to pull out the KSP from the SNES:<br>
<br>
<a href="http://www-unix.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/SNES/SNESGetKSP.html">http://www-unix.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/SNES/SNESGetKSP.html
</a><br>
<br>
and then flip the flag in the KSP:<br>
<br>
<a href="http://www-unix.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/KSP/KSPSetInitialGuessNonzero.html">http://www-unix.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/KSP/KSPSetInitialGuessNonzero.html
</a><br>
</div><br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Second: I see that PETSc checks the preconditioner matrix for positive<br>definiteness, but does it check whether the current solution iterate is
<br>positive def. (i.e. if x_i is the current solution iterate and A is the<br>current Jacobian matrix, does the algorithm check if (x_i)^T * A * (x_i)<br> > 0)?</blockquote><div><br>
We cannot check that the preconditioner is positive definite. What we actually<br>
check it that the A-inner product calculated at each iteration is positive.<br>
<br>
Matt<br>
</div><br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Thanks so much for your help.<br><br>Best Regards,<br><br>Paul<br>
</blockquote></div>-- <br>"Failure has a thousand explanations. Success doesn't need one" -- Sir Alec Guiness