[petsc-users] Galerkin multigrid coarsening

[Kaus, Boris]

Thanks Jed,

I'll give it a try and let you know. Might take a bit though.


Boris 

> On Jan 30, 2014, at 17:45, "Jed Brown" <jed at jedbrown.org> wrote:
> 
> Boris Kaus <kaus at uni-mainz.de> writes:
> 
>> While doing multigrid it is possible to explicitly set restriction and
>> prolongation/interpolation operators in PETSC using PCMGSetRestriction
>> and PCMGSetInterpolation.  In many cases, the restriction (R) and
>> prolongation (P) operators are simply the transpose of each other
>> (R=P’).
>> 
>> Yet, in certain cases, for example variable viscosity Stokes with a
>> staggered finite difference discretization, it is advantageous if they
>> are different (see http://arxiv.org/pdf/1308.4605.pdf).
>> 
>> Accordding to the Wesseling textbook, Galerkin coarsening of the fine
>> grid matrix is defined as:
>> 
>> Acoarse = R*A*P
>> 
>> Yet, as far as I can tell, PETSc seems to implement this as:
>> 
>> Acoarse = R*A*R’
> 
> Certainly, the code just had an outdated assumption.  I think I have
> added support for this (it does R*A*P when R!=P), but I don't have a
> test case readily available.  Could you try branch
> 'jed/pcmg-galerkin-rap' (if you were using v3.4/maint) or 'next' if
> you've been tracking petsc-dev?