On Tue, Dec 13, 2011 at 10:48 AM, Uwe Schlifkowitz <span dir="ltr"><<a href="mailto:uwe.schlifkowitz@uibk.ac.at">uwe.schlifkowitz@uibk.ac.at</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">
On 13.12.2011, at 16:43, Matthew Knepley wrote:<br>
> On Tue, Dec 13, 2011 at 9:36 AM, Uwe Schlifkowitz <<br>
> <a href="mailto:uwe.schlifkowitz@uibk.ac.at">uwe.schlifkowitz@uibk.ac.at</a>> wrote:<br>
><br>
>> On 12.12.2011, at 20:05, Barry Smith wrote:<br>
>> problem as described here:<br>
>> <a href="http://lists.mcs.anl.gov/pipermail/petsc-users/2011-August/009571.html" target="_blank">http://lists.mcs.anl.gov/pipermail/petsc-users/2011-August/009571.html</a><br>
>> possible, so Clemens (and I) resort to MATSOLVERPETSC to obtain D.<br>
>> if you really need it, but what are you using it for? Do you want the<br>
>> inertia? We could give that for PETSc factored matrices.<br>
>><br>
>> K is an element stiffness matrix. K is related to displacement d and force<br>
>> f by K d = f. Since K is not invertible, a solution can be found by means<br>
>> of LDL^T decomposition where K = LDL^T .<br>
>><br>
>> Here L is the lower triangular matrix and D is the diagonal matrix, which<br>
>> is what I am looking for. The number of negative values in D correspond to<br>
>> the number of negative eigenvalues of K and serve as an indicator of the<br>
>> occurrence of branching points.<br>
>><br>
>> I am not sure if this has to do with inertia, but I am fairly new to all<br>
>> of this so suggestions are very welcome.<br>
><br>
><br>
> Yes, it looks like what you want<br>
><br>
> <a href="http://en.wikipedia.org/wiki/Sylvester's_law_of_inertia" target="_blank">http://en.wikipedia.org/wiki/Sylvester's_law_of_inertia</a><br>
><br>
> Matt<br>
><br>
<br>
I found MatGetInertia in petsc's documentation and i am not sure how to use it. I found an old example from petsc-2.3.1 (<a href="http://www.mcs.anl.gov/petsc/petsc-2.3.1/src/ksp/ksp/examples/tutorials/ex10.c.html" target="_blank">http://www.mcs.anl.gov/petsc/petsc-2.3.1/src/ksp/ksp/examples/tutorials/ex10.c.html</a> ), but things seem to have changed since that version. Can you give an example?<br>
</blockquote><div><br></div><div>It works exactly as in this example.</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">
><br>
>><br>
>> Uwe<br>
<br>
<br>
<br>
</blockquote></div><br><br clear="all"><div><br></div>-- <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>