[petsc-users] Return value of PCFactorGetMatrix

[Matthew Knepley]

On Tue, Dec 13, 2011 at 9:36 AM, Uwe Schlifkowitz <
uwe.schlifkowitz at uibk.ac.at> wrote:

> On 12.12.2011, at 20:05, Barry Smith wrote:
> >
> > On Dec 12, 2011, at 11:49 AM, Uwe Schlifkowitz wrote:
> >
> >> I'm continuing Clemens Domanig's work. Currently I'm stuck at his
> problem as described here:
> http://lists.mcs.anl.gov/pipermail/petsc-users/2011-August/009571.html
> >>
> >> As far as I understand, getting the diagonal matrix from MUMPS is not
> possible, so Clemens (and I) resort to MATSOLVERPETSC to obtain D.
> >
> >   We could add support for MatGetDiagonal() for PETSc factored matrices
> if you really need it, but what are you using it for? Do you want the
> inertia? We could give that for PETSc factored matrices.
> >
> >
> >   Barry
> >
>
> K is an element stiffness matrix. K is related to displacement d and force
> f by K d = f. Since K is not invertible, a solution can be found by means
> of LDL^T decomposition where K = LDL^T .
>
> Here L is the lower triangular matrix and D is the diagonal matrix, which
> is what I am looking for. The number of negative values in D correspond to
> the number of negative eigenvalues of K and serve as an indicator of the
> occurrence of branching points.
>
> I am not sure if this has to do with inertia, but I am fairly new to all
> of this so suggestions are very welcome.


Yes, it looks like what you want

  http://en.wikipedia.org/wiki/Sylvester's_law_of_inertia

    Matt


>
> Uwe




-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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