[petsc-users] Nullspaces

[Matthew Knepley]

On Tue, Dec 7, 2021 at 11:19 AM Marco Cisternino <
marco.cisternino at optimad.it> wrote:

> Good morning,
>
> I’m still struggling with the Poisson equation with Neumann BCs.
>
> I discretize the equation by finite volume method and I divide every line
> of the linear system by the volume of the cell. I could avoid this
> division, but I’m trying to understand.
>
> My mesh is not uniform, i.e. cells have different volumes (it is an octree
> mesh).
>
> Moreover, in my computational domain there are 2 separated sub-domains.
>
> I build the null space and then I use MatNullSpaceTest to check it.
>
>
>
> If I do this:
>
> MatNullSpaceCreate(getCommunicator(), PETSC_TRUE, 0, nullptr, &nullspace);
>
> It works
>

This produces the normalized constant vector.


> If I do this:
>
> Vec nsp;
>
> VecDuplicate(m_rhs, &nsp);
>
> VecSet(nsp,1.0);
>
> VecNormalize(nsp, nullptr);
>
> MatNullSpaceCreate(getCommunicator(), PETSC_FALSE, 1, &nsp, &nullspace);
>
> It does not work
>

This is also the normalized constant vector.

So you are saying that these two vectors give different results with
MatNullSpaceTest()?
Something must be wrong in the code. Can you send a minimal example of
this? I will go
through and debug it.

  Thanks,

     Matt


> Probably, I have wrong expectations, but should not it be the same?
>
>
>
> Thanks
>
>
>
> Marco Cisternino, PhD
> marco.cisternino at optimad.it
>
> ______________________
>
> Optimad Engineering Srl
>
> Via Bligny 5, Torino, Italia.
> +3901119719782
> 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.cse.buffalo.edu/~knepley/>
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