[petsc-users] help needed for -snes_mf_operator

[Dominik Szczerba]

I am successfully solving a couple (unstructured FEM) benchmark
problems with SNES, providing both FormFunction and FormJacobian. I am
using a simple (polynomial) form for the nonlinear part so I am fairly
sure I compute the Jacobian correctly and exactly. I am also getting
the expected results.

Now I want to reproduce the same results without providing the
Jacobian, preparing for future real life cases where it will not be
known. I am under the impression that Petsc can approximate the
Jacobian itself using some sort of secant method. How exactly am I
supposed to use it? If I change nothing in the code, just pass
-snes_mf_operator to the options, the solver stagnates. The same if I
fill the Jacobian with zeros. The same if only with A. What am I
supposed to do without the known Jacobian to trigger its
approximation?

Thanks a lot
Dominik