[petsc-users] how is snes_test_jacobian_view computing the matrix difference?

[Stefano Zampini]

Maybe you are using a DMDA, with one matrix coming from it and the other
not? DMDA changes the View to dump in natural ordering.

Il giorno lun 2 mar 2026 alle ore 18:10 Matteo Semplice via petsc-users <
petsc-users at mcs.anl.gov> ha scritto:

> Dear all,
>
>     the following output from -snes_test_jacobian_view lokks clear enough
> to pinpoint my mistake (I must be putting jacobian elements in the wrong
> place), but it nevertheless left me suprised.
> 0 SNES Function norm 2.945516399303e-01
>   ---------- Testing Jacobian -------------
>   Testing hand-coded Jacobian, if (for double precision runs) ||J -
> Jfd||_F/||J||_F is
>     O(1.e-8), the hand-coded Jacobian is probably correct.
>   ||J - Jfd||_F/||J||_F = 0.000121529, ||J - Jfd||_F = 0.395752
>   Hand-coded Jacobian ----------
> Mat Object: 4 MPI processes
>   row 84:     (84, 60.)      (85, 0.)      (3446, -80.)      (3447, 0.)
>   (6808, 20.)      (6809, 0.)
>   row 85:     (84, 0.00914434)      (85, 1.)      (3446, 0.)      (3447,
> 0.)      (6808, 0.)      (6809, 0.)
>
>    Finite difference Jacobian ----------
> Mat Object: 4 MPI processes
>   type: mpiaij
>   row 84:   (84, 60.)    (1806, -80.)    (3528, 20.)
>   row 85:   (85, 0.999971)
>
>   Hand-coded minus finite-difference Jacobian with tolerance 1e-05
> ----------
> Mat Object: 4 MPI processes
>   type: mpiaij
>   row 84:
>   row 85:   (84, 0.00914434)    (85, 2.9293e-05)
>
> The difference of the two matrices shoudl be nonzero in row 84 for all
> elements 1806, 3528, 3446, 6808. Is it intended that the difference is
> shown only for matrix elements that are present in both the hand-coded and
> the FD jacobian?
>
> I would have expected an output like
>
> row 84: (1806,80.)   (3446,-80.), etc
>
> (This was on  PETSc Release Version 3.24.2)
>
> Thanks
>
>     Matteo
>
> --
> Prof. Matteo Semplice
> Università degli Studi dell’Insubria
> Dipartimento di Scienza e Alta Tecnologia – DiSAT
> Professore Associato
> Via Valleggio, 11 – 22100 Como (CO) – Italia
> tel.: +39 031 2386316
>
>

-- 
Stefano
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