[petsc-users] Create MATSEQAIJ from MATMPIAIJ

[Matthew Knepley]

On Mon, Jun 22, 2020 at 7:14 AM Eda Oktay <eda.oktay at metu.edu.tr> wrote:

> Hello everyone,
>
> I am trying to find elements in off diagonal blocks of a parallel
> sparse matrix. That is why, I want to use MatGetDiagonalBlock and from
> the matrix I obtain, I want to obtain off diagonal elements by
> subtracting it from my original matrix by using MatAXPY.
>
> However, since MatGetDiagonalBlock gives a sequential matrix and my
> original one is parallel, I can't use MatAXPY. That's why I want to
> change the type of one of the matrices.
>
> How can I change a MATSEQAIJ to MATMPIAIJ or vice versa?
>

I assume you want a parallel matrix with the element in the diagonal block
removed. I can think
of at least two ways to do this which sound easier to me:

  1) Make a copy and then zero out the diagonal block is a way similar to
MatChop:
https://www.mcs.anl.gov/petsc/petsc-current/src/mat/utils/axpy.c.html#MatChop

  2) Use MatGetSubMatrix() and exclude any columns from the diagonal block
on each process.

  Thanks,

     Matt


> Thanks!
>


-- 
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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