On Mon, Sep 23, 2024 at 5:33 AM Matteo Semplice via petsc-users < petsc-users@mcs.anl.gov> 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/DMPlexGetClosureIndices/__;!!G_uCfscf7eWS!fDvGoz5OloH7xryMIAIqyYsslt8U4HqegW7HeYDlwRPbkTloMU6k0kze8NlG7S2DxDuY2nUXRQcBIiuqLU1o$ which is what is used inside of https://urldefense.us/v3/__https://petsc.org/main/manualpages/DMPlex/DMPlexMatSetClosure/__;!!G_uCfscf7eWS!fDvGoz5OloH7xryMIAIqyYsslt8U4HqegW7HeYDlwRPbkTloMU6k0kze8NlG7S2DxDuY2nUXRQcBIi82N3yY$ 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_uCfscf7eWS!fDvGoz5OloH7xryMIAIqyYsslt8U4HqegW7HeYDlwRPbkTloMU6k0kze8NlG7S2DxDuY2nUXRQcBIjjfDigI$ <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!fDvGoz5OloH7xryMIAIqyYsslt8U4HqegW7HeYDlwRPbkTloMU6k0kze8NlG7S2DxDuY2nUXRQcBIpAwlPEi$ >
