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

[Matthew Knepley]

On Tue, Jul 8, 2014 at 8:28 AM, Andrew Cramer <andrewdalecramer at gmail.com>
wrote:

> Doing a basic cantilever bending problem to test my code which results in
> the linear system Au = b.
>
>  - Using DMDA for the domain and KSPSetComputeOperators
>  - Solving it using various methods in petsc gets similar (within 1%)
> solutions
>  - Even using -pc_type lu
>  - Using KSPGetOperators and KSPGetRhs to export to matlab
>
> Exporting the matrix and and the rhs, importing them into matlab  and
> solving with backslash gives a solution which matches the Euler-Bernoulli
> beam model much closer (0.4% error vs 9.6%).
>

As Karl says, this is probably truncation error, but more to the point this
is accidental. You would not expect
a solution that is more accurate than the discretization error. If you are
comparing a discrete solution to a
continuous solution, you want to look at "mesh convergence" meaning that
you run on a series of refined
meshes, calculate the error, and usually plot log(error) against mesh size
h.

    Matt


> Calculating the residual of petsc's solution using matlab (
> norm(A*u-b)/norm(b)) I get 0.3 having solved with -pc_type lu.
>
> Is there a way I could have accidentally made petsc solve a different
> problem to Ax=b? I've been looking at this code for a while now (days) and
> can't seem to figure out what is wrong.
>



-- 
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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