[petsc-users] Matlab gets a different solution to Petsc

[Jed Brown]

Andrew Cramer <andrewdalecramer at gmail.com> writes:

> I get *significantly *different results from backslash and petsc using
> the *same
> matrix*.
>
>                  Deflection  |Au-b|/|b|
> Euler-Bernoulli:       4000
> Petsc:                 4386        0.3
> Matlab:                4013    1.6e-10
>
> I'm exporting to 6 significant figures. Condition number is rather poor
> (~2e7) but I get similar or worse results with problems having lower
> conditions numbers (~1e3).

Are you sure the matrices are the same?  Use "-ksp_view_mat binary",
then in MATLAB, A = PetscBinaryRead('binaryoutput') if you want to get
both matrices in the same environment.  Or explicitly compute the norms
in PETSc.

To debug, start with the simplest and most well-conditioned problem
that exhibits the issue.
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