Jed, It is ok now!
I have another question. I use the Discontinoues Galerkin discretization for the equations of gas dynamics. I use a matrix free formulation and that's why I need a very good preconditioner, which I compute it numerically and untill now it works ok, but it is a bit slow The preconditioner is just the Jacobian of the system. I wonder if PETSc has a built in capability to compute its entries. Costas
