semi coarsening with DA's

[Matthew Knepley]

On Thu, Jul 17, 2008 at 8:03 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
> On Jul 16, 2008, at 10:16 PM, Dave May wrote:
>
>> Hello,
>>    Does anyone know the best way to perform semi coarsening using DA's?
>> I was trying to use the following grid sequence
>>  level [2]: 20 x 20
>>  level [1]: 11 x 11
>>  level [0]: 6 x 6
>> where the numbers M x N corresponds to the number of nodes in the x and y
>> direction respectively.
>> However I was unable to generate the interpolation operator using
>>  DAGetInterpolation(DA dac,DA daf,Mat *A,Vec *scale)
>> Doing so caused the following error
>>  [0]PETSC ERROR: Arguments are incompatible!
>>  [0]PETSC ERROR: Ratio between levels: (mx - 1)/(Mx - 1) must be integer:
>> mx 20 Mx 11!
>
> In this case mx = 21, Mx = 11 gives (21 - 1)/(11-1) = 2 works.
>>
>>
>> Is there a more general way to construct the interpolation operator
>> within petsc for this type of grid sequence?
>
>  What I usually do is start with a coarse DA, then use DARefine() to
> generate the finer
> ones, this makes sure the interpolation will work. You can use
> DASetRefinementFactor()
> to control which directions get refined.

DAve's question is different. He is trying to use DA for doing staggered methods
for Stokes where both A and B^T B are solved with MG, meaning that he cannot
adjust grid sizes like that. Is there a way to specify coarse grids
himself, but have
PETSc calculate the interpolation operators automatically? I would
look, but I am
on travel :)

  Matt

>   Barry
>
>>
>>
>> Thanks,
>>  Dave.
-- 
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