On Mon, Sep 23, 2024 at 5:33 AM Matteo Semplice via petsc-users < [email protected]> wrote:

Dear petsc,

I need to hand-code a jacobian and I can't figure out how to translate the DMplex points/fields to matrix indices.

My DMPlex has a section with m fields per cell (which makes for n>m dof per cell since some fields are vector). Say I want to insert an nxn block for the row corresponding to cell c and coloumn of its neighbour d. I guess that I should call either MatSetValues/MatSetValuesLocal or the blocked variants, but how do I find the row/col indices to pass in starting from the c/d dmplex points? And, while I am at it, which MatSetValues version standard/Local standard/blocked is best?

I looked for a petsc example, but fails to find it: if there's one, can you just point me to it?

1. In FEM, the main unit of indices is usually the closure, so we provide https://urldefense.us/v3/__https://petsc.org/main/manualpages/DMPlex/DMPlexG... which is what is used inside of https://urldefense.us/v3/__https://petsc.org/main/manualpages/DMPlex/DMPlexM... 2. These indices are calculated using the global section, so you can just DMGetGlobalSection(dm, &gs); and then use PetscSectionGetDof() and PetscSectionGetOffset(), knowing that off-process values are encoded as -(off + 1), so you need to convert those. Does this make sense? Thanks, Matt

Thanks in advance.

Matteo Semplice

-- 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://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/__;fg!!G_uCf... <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfs... >