[petsc-users] Replace schur solver in fieldsplit by a matrix multiplication

[Thomas Witkowski]

Is there a simple way to replace the Schur ksp in fieldsplit 
preconditioner by a matrix multiplication? So, instead of solving with 
some Schur operator approximation I just want to make a matrix-vector 
multiplication, where the matrix is a sparse approximation of the 
inverse of the Schur complement. Thanks for any suggestions!

Thomas