Matthew Knepley <knep...@gmail.com> writes: > On Thu, Oct 22, 2015 at 9:37 AM, Marco Zocca <zocca.ma...@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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