[petsc-users] Any changes in ML usage between 3.1-p8 -> 3.3-p6?

[Matthew Knepley]

On Wed, Apr 17, 2013 at 4:26 PM, Jozsef Bakosi <jbakosi at lanl.gov> wrote:

>
> > Mark F. Adams mark.adams at columbia.edu
> > Wed Apr 17 14:25:04 CDT 2013
> >
> > 2) If you get "Indefinite PC" (I am guessing from using CG) it is
> because the
> > preconditioner
> >     really is indefinite (or possible non-symmetric). We improved the
> checking
> >     for this in one
> >     of those releases.
> >
> > AMG does not guarantee an SPD preconditioner so why persist in trying to
> use
> > CG?
> >
> >
> > AMG is positive if everything is working correctly.
> >
> > Are these problems only semidefinite?  Singular systems can give erratic
> > behavior.
>
> It is a Laplace operator from Galerkin finite elements. And the PC is fine
> on
> ranks 1, 2, 3, and 5 -- indefinite only on 4. I think we can safely say
> that the
> same PC should be positive on 4 as well.
>

Why is it safe? Because it sounds plausible? Mathematics is replete with
things
that sound plausible and are false. Are there proofs that suggest this? Is
there
computational evidence? Why would I believe you?

   Matt


> Can you guys please CC jbakosi at lanl.gov? Thanks, J
>



-- 
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20130417/76ebfc16/attachment.html>