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

[Barry Smith]

  At first glance the output is surprising. I believe your understanding is correct and something is going very wrong to produce the output you report. 

  Is there any way you can share the code that produces this output (if you send it to petsc-maint at mcs.anl.gov the code will never be shared outside the PETSc developers who monitor that email).

   Barry


> On Mar 2, 2026, at 10:09 AM, Matteo Semplice via petsc-users <petsc-users at mcs.anl.gov> wrote:
> 
> 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

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