[petsc-dev] FD matrices

[Mark Adams]

>
> Okay, there is
>
>   -snes_fd_color -snes_fd_color_use_mat -mat_coloring_type greedy
>
> which takes a properly preallocated matrix.
>

Which Mat will it use?  Our operators matrix is a shell so it must be the PC ...

If it uses the PC Mat we could take the transpose of the PC Mat, scale
it by like 1.e-30, add it back into the PC Mat to get a symmetric
structure, if that matters, and get a safe graph for coloring.

Dan: this is easy, I will send you code today to modify "A", just use
Matt's parameters above.