[petsc-users] Incorrect Eigenvalues when Setting KSP and PC types

[Greg Meyer]

Yes, they are Hermitian.

On Thu, Sep 21, 2017 at 3:43 PM Hong <hzhang at mcs.anl.gov> wrote:

> Greg:
>
> OK, the difference is whether LU or Cholesky factorization is used. But I
>> would hope that neither one should give incorrect eigenvalues, and when I
>> run with the latter it does!
>>
> Are your matrices symmetric/Hermitian?
> Hong
>
>>
>> On Thu, Sep 21, 2017 at 2:05 PM Hong <hzhang at mcs.anl.gov> wrote:
>>
>>> Gregory :
>>> Use '-eps_view' for both runs to check the algorithms being used.
>>> Hong
>>>
>>> Hi all,
>>>>
>>>> I'm using shift-invert with EPS to solve for eigenvalues. I find that
>>>> if I do only
>>>>
>>>> ...
>>>>   ierr = EPSGetST(eps,&st);CHKERRQ(ierr);
>>>>   ierr = STSetType(st,STSINVERT);CHKERRQ(ierr);
>>>> ...
>>>>
>>>> in my code I get correct eigenvalues. But if I do
>>>>
>>>> ...
>>>>   ierr = EPSGetST(eps,&st);CHKERRQ(ierr);
>>>>   ierr = STSetType(st,STSINVERT);CHKERRQ(ierr);
>>>>   ierr = STGetKSP(st,&ksp);CHKERRQ(ierr);
>>>>   ierr = KSPGetPC(ksp,&pc);CHKERRQ(ierr);
>>>>   ierr = KSPSetType(ksp,KSPPREONLY);CHKERRQ(ierr);
>>>>   ierr = PCSetType(pc,PCCHOLESKY);CHKERRQ(ierr);
>>>> ...
>>>>
>>>> the eigenvalues found by EPS are completely wrong! Somehow I thought I
>>>> was supposed to do the latter, from the examples etc, but I guess that was
>>>> not correct? I attach the full piece of test code and a test matrix.
>>>>
>>>> Best,
>>>> Greg
>>>>
>>>
>>>
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