[petsc-users] on SNESSetJacobian semantics

[Jed Brown]

Matthew Knepley <knepley at gmail.com> writes:

> On Thu, Oct 22, 2015 at 9:37 AM, Marco Zocca <zocca.marco at gmail.com> wrote:
>
>> Thank you, but this is not really helping;
>>
>> why are there two slots to supply the Jacobian of the same function, i.e.
>> Amat and J ?
>>
>
> Amat is just the storage, not the values. J computes the values.

More precisely, Amat provides the action of the Jacobian (possibly
matrix-free).

Pmat provides the data from which to construct the preconditioner.  This
is most frequently an assembled matrix (possibly from a simpler
discretization, skipping some terms, etc.), but could hold additional
information or eschew matrix entries entirely.
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