[petsc-users] Return value of PCFactorGetMatrix

[Uwe Schlifkowitz]

On 13.12.2011, at 16:43, Matthew Knepley wrote:
> 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:
>> problem as described here:
>> http://lists.mcs.anl.gov/pipermail/petsc-users/2011-August/009571.html
>> possible, so Clemens (and I) resort to MATSOLVERPETSC to obtain D.
>> 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.
>> 
>> 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
> 

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 (http://www.mcs.anl.gov/petsc/petsc-2.3.1/src/ksp/ksp/examples/tutorials/ex10.c.html ), but things seem to have changed since that version. Can you give an example?


> 
>> 
>> Uwe