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

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:
>> Actually, the problem is more subtle because if one wants an SPD
>> multigrid cycle, they only need that the post-smoother is the transpose
>> of the pre-smoother.  
>
>    Hmm. Is this true? This guy’s matrix causes grief with
>    indefiniteness but since it uses SSOR smoother I think that
>    post-smooth should have been the transpose of the pre smooth
>    automatically?

Yes, S_pre = S_post = S is symmetric so S_pre^T = S_post trivially.  But
S appears to be indefinite.  It's simpler to understand indefiniteness
for symmetric matrices (like SSOR), but we shouldn't get overzealous and
put in checks for whether S is SPD.
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