[petsc-dev] [petsc-users] unreliable AMG in PETSc

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:

>    Well, we did report the entire preconditioner was indefinite, but not what part of the preconditioner caused it to be indefinite. 
>
>    Could run the smoother with CG for many steps and see if that reports indefiniteness. Would it, I’m not even sure of that.

I think it would report indefiniteness, but it depends on the RHS vector/initial guess.

>    We could easily add a test MatIsNonsingularM() i.e. a_ii > =0 a_ij <= 0, useful information but I don’t think it helps the user “fix” the problem.

If they actually have an M-matrix, everything should just work.  So how
to help them when they have a problem/formulation that produces a matrix
where methods aren't working?
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