[petsc-users] A question on finite difference Jacobian

[Jed Brown]

"Zou (Non-US), Ling" <ling.zou at inl.gov> writes:

> Hi All,
>
> If the non-zero pattern of a finite difference Jacobian needs 20 colors to
> color it (20 comes from MatFDColoringView, the non-zero pattern is
> pre-determined from mesh connectivity), is it true that PETSc needs 40
> functions evaluation to get the full Jacobian matrix filled? This is
> because that a perturbation per color needs two function evaluation
> according to PETSc manual (ver 3.6, page 123, equations shown in the middle
> of the page).

Note that F(u) has already been evaluated and is the same for all the
perturbation directions.  A centered difference would require two
evaluations per direction, as you say.

> But I only see 20 function evaluations. I probably have some
> misunderstanding somewhere. Any suggestions?
>
> Ling
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