[petsc-users] Multigrid preconditioning

[Michele Rosso]

Hi,

I am solving a variable coefficients Poisson equation with periodic BCs.
The equation is discretized by using the standard 5-points stencil 
finite differencing scheme.
I managed to solve the system successfullywith the PCG method and now I 
would like to add
a preconditioner to speed up the calculation. My idea is to use the 
multigrid preconditioner.

Example ex22f.F implements what I think I need.
If I understand correctly example ex22f.F, the subroutines "ComputeRHS" 
and "ComputeMatrix" define how the
matrix and rhs-vector have to be computed at each level.
In my case tough, both the jacobian and the rhs-vector cannot be 
computed "analytically", that is, they depend on variables
whose values are available only at the finest grid.

How can I overcome this difficulty?

Thank you,

Michele


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