[petsc-users] question on PETSc option '-snes_mf_operator'

[Matthew Knepley]

On Thu, Sep 27, 2012 at 2:01 PM, Zou (Non-US), Ling <ling.zou at inl.gov>wrote:

> Dear all,
>
> I have a question on the PETSc option '-snes_mf_operator'.
> I am reading the    <PETSc Users Manual Revision 3.3>. On page 100, 2nd
> paragraph, it says:
>
>
> ==============================================================================
> "However, it allows us to check the analytic Jacobian we construct in
> FormJacobian() by passing the -snes_mf_operator flag. This causes PETSc to
> approximate the Jacobian using finite differencing of the function
> evaluation (discussed in section 5.6), and the analytic Jacobian becomes
> merely the preconditioner."
>
> ==============================================================================
>
> I wonder, if the '-snes_mf_operator' option is used, the Jacobian will
> always be calculated from the finite difference method while
> ignore whatever has been provided from user.
>
Yes, yhe action will be FD, but it will take the user provided operator to
form a preconditioner from.

   Matt


> One more question, if I pass '-snes' to PETSc, since it is direct Newton's
> method, I assume it will explicitly construct a Jacobian. Does this
> Jacobian come from user provided Jacobian or from the finite difference
> Jacobian?
>
>
> Thanks,
>
>
> Ling
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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