[petsc-users] Elliptic operator with Neumann conditions

[Marco Cisternino]

I'm sorry Matt but I cannot understand what flexible gmres computes when no null space is created.
Could you give me some hints, please? Even in very simple cases...
Thanks


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 19:38:08
A: Marco Cisternino
Cc: petsc-users
Oggetto: Re: [petsc-users] Elliptic operator with Neumann conditions

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


________________________________________
Da: Matthew Knepley <knepley at gmail.com<mailto: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><mailto: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><mailto:marco.cisternino at optimad.it<mailto:marco.cisternino at optimad.it>>
_______________________________
OPTIMAD Engineering srl
Via Giacinto Collegno 18, Torino, Italia.
+3901119719782<tel:%2B3901119719782><tel:%2B3901119719782>
www.optimad.it<http://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/>