[petsc-users] Elliptic operator with Neumann conditions

[Matthew Knepley]

On Thu, Feb 8, 2018 at 5:29 AM, Marco Cisternino <
marco.cisternino at optimad.it> wrote:

> Thanks Matt,
> I'm using KSPFGMRES and I'm sorry but what do you mean with initial
> residual?
> I also force a non-zero initial guess.
>

If your initial residual has a component in the null space of the operator,
it is likely to stay there.

   Matt


> Thanks again
>
>
> Marco Cisternino, PhD
> marco.cisternino at optimad.it
> _______________________________
> OPTIMAD Engineering srl
> Via Giacinto Collegno 18, Torino, Italia.
> +3901119719782
> www.optimad.it
>
>
> ________________________________________
> Da: Matthew Knepley <knepley at gmail.com>
> Inviato: mercoledì 7 febbraio 2018 18:57:56
> A: Marco Cisternino
> Cc: petsc-users
> Oggetto: Re: [petsc-users] Elliptic operator with Neumann conditions
>
> On Thu, Feb 8, 2018 at 4:43 AM, Marco Cisternino <
> marco.cisternino at optimad.it<mailto:marco.cisternino at optimad.it>> wrote:
> Hi everybody,
> I would like to ask what solution is computed if I try to solve the linear
> system relative to the problem in subject without creating the null space.
> I tried with and without the call to
> MatNullSpaceCreate(m_communicator, PETSC_TRUE, 0, NULL, &nullspace);
> and I get zero averaged solution with and the same solution plus a
> constant without.
> How does PETSc  work in the second case?
>
> It depends on the Krylov method you use and the initial residual. We do
> not do anything special.
>
>   Thanks,
>
>      Matt
>
> Does it check the matrix singularity? And is it able to create the null
> space with the constant automatically?
> Thanks.
>
>
> Marco Cisternino, PhD
> marco.cisternino at optimad.it<mailto:marco.cisternino at optimad.it>
> _______________________________
> OPTIMAD Engineering srl
> Via Giacinto Collegno 18, Torino, Italia.
> +3901119719782<tel:%2B3901119719782>
> www.optimad.it<http://www.optimad.it>
>
>
>
>
> --
> 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
>
> https://www.cse.buffalo.edu/~knepley/<http://www.caam.rice.edu/~mk51/>
>



-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
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