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

[Andrew Cramer]

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%).

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.
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