[petsc-users] Preconditioning in matrix-free methods

[Jed Brown]

Konstantinos Kontzialis <ckontzialis at lycos.com> writes:
>     Thank you for your response. I understand what you have written, but 
> maybe I'm missing something else . I have
> not understood why PETSC is giving me this error. With -snes_mf I get my 
> code running (however many gmres iterations
> are needed and convergence sometimes stalls), but with -snes_mf_operator 
> I still get the error I posted on the list. What
> should I do to overcome it? Am I missing something? Is there anything I 
> do wrong?

You didn't set an (approximate) Jacobian.  If you want to use
preconditioning, you either need to build a matrix (possibly with
-snes_fd_color) or write your own custom preconditioner.
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