[petsc-users] GAMG and zero pivots follow up

[Mark Adams]

On Fri, Nov 13, 2015 at 2:48 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Fri, Nov 13, 2015 at 12:28 PM, Jed Brown <jed at jedbrown.org> wrote:
>
>> Matthew Knepley <knepley at gmail.com> writes:
>> > Something very strange is happening here. CG should converge
>> monotonically,
>> > but above it does not. What could be happening?
>>
>> Are you use -ksp_norm_type natural?  CG is not monotone in other norms.
>>
>
> Yikes! I did not check that. Why do we have PRECONDITIONED as the default
> for CG?
>
>
The two norm of the residual (ie, natural, right?) is not monotone either
in CG.


>    Matt
>
> Also, if boundary conditions are enforced using a nonsymmetric
>> formulation (for example), then you can get lack of monotonicity with CG
>> that may not be catastrophic.
>>
>
>
>
> --
> 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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