On Sat, Feb 11, 2012 at 00:00, recrusader <recrusader@gmail.com> wrote:

in http://www.mcs.anl.gov/petsc/petsc-dev/src/ksp/ksp/examples/tutorials/ex43.c.html

Why are two matrices set to MATAIJ? if I set MATAIJCUSP, is them changed?

1468: DMCreateMatrix(da_Stokes,MATAIJ,&A);
1469:   DMCreateMatrix(da_Stokes,MATAIJ,&B);

You have to follow the code down to where those matrices are used.

1478: AssembleA_Stokes(A,da_Stokes,da_prop,properties);

1479: AssembleA_PCStokes(B,da_Stokes,da_prop,properties);

This uses a "block preconditioner", the B matrix is intentionally different from the A matrix in order to handle the saddle point.