[petsc-dev] [petsc-users] About the interpolation and restriction matrix for cell-centered multigrid.

[Jed Brown]

ex18 has the same issue, but this aspect can probably be fixed by setting DMDA_Q0.

Patrick Sanan <patrick.sanan at gmail.com> 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 <jed at jedbrown.org>:
>
>> Mark Adams <mfadams at lbl.gov> writes:
>>
>> > 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.
>>
>> > 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.
>>