[petsc-users] Trying to write Fortran version of ksp/ex34.c

[Matthew Knepley]

On Thu, Apr 3, 2014 at 8:43 AM, Åsmund Ervik <asmund.ervik at ntnu.no> wrote:

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> Dear PETSc users,
>
> I'm trying to write a Poisson solver in Fortran using the
> KSPSetComputeOperators etc. framework. When debugging this, I ended up
> modifying ksp/ex22.f so that it matches ksp/ex34.c. The difference
> between these is that ex34.c has a non-constant RHS and Neumann BCs,
> which is closer to what I want.
>

If it has Neumann conditions, then it has a null space. Have you included
this in
your solver? That can cause a residual offset.

  Matt


> Now, when I run these two programs I get the following:
>
> ./ex22f_mod
>    Residual, L2 norm         1.007312
>    Error, sup norm         0.020941
>    Error, L1 norm        10.687882
>    Error, L2 norm         0.340425
>
> ./ex34
>    Residual, L2 norm 1.07124e-05
>    Error, sup norm 0.0209405
>    Error, L1 norm 0.00618512
>    Error, L2 norm 0.000197005
>
> but when I MatView/VecView the matrix and RHS for each example (write
> them to file), there is no difference. I have attached ex22f_mod.F90,
> any suggestions on what the error is?
>
> Once this example works, you can of course include it with PETSc.
>
> Regards,
> Åsmund
>
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-- 
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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