ex18 has the same issue, but this aspect can probably be fixed by setting DMDA_Q0. Patrick Sanan <[email protected]> writes:

This sort of feedback is great in terms of learning what can move out of "tutorials" and into "tests". https://gitlab.com/petsc/petsc/-/merge_requests/2629

Am Di., 24. März 2020 um 16:22 Uhr schrieb Jed Brown <[email protected]>:

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Mark Adams <[email protected]> writes:

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Good question. It does look like there is Q1:

src/dm/impls/da/da.c:- ctype - DMDA_Q1 and DMDA_Q0 are currently the only supported forms

And in looking at a cell centered example src/snes/examples/tutorials/ex20.c, it looks like only DMDA_Q1 works. I get an error when I set it to DMDA_Q0 (DMDA_Q1 is the default). This is puzzling, Q0 is natural in cell centered.

The comments in those examples are kinda wrong -- they never told the DM it was cell-centered so it uses a multigrid that isn't compatible with the boundary conditions. The interpolation is Q1 on the dual grid, not conservative Q1 on cells.

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I am not familiar with DMDA and I don't understand why, from ex20, that you have an odd number of points on a cell centered grid and an even number for vertex centered (eg, ex14). I would think that it should be the opposite.

The example is bad.