[petsc-users] GAMG

[Mark Adams]

>
>
>>
> AMG (the agglomeration kind) needs to know the near null space of your
> operator in order
> to work. You have an elasticity problem (I think), and if you take that
> operator without boundary
> conditions, the energy is invariant to translations and rotations. The
> space of translations and
> rotations is a 6D space (3 translations, 3 rotations). You need to express
> these in the basis for
> your problem (I assume linear elements, P1).
>

Actually, these vectors are purely geometric. If these rigid body modes are
not your kernel then you have a bad discretization or you are not doing 3D
elasticity.

Anyway, this reminds me that the problem goes away w/o the RBMs.  The fine
grid eigen estimate was large and will not be affected by the null space
business. The second grid had a huge eigenvalue and that could be affected
by the null space.

What is your Poisson ratio?


> This is what PCSetCoordinates() tries to do. Something
> is going wrong, but its hard for us to say what since I have no idea what
> your problem looks like.
> So you can make these vectors yourself and provide them to GAMG using
> MatSetNearNullSpace().
>
>    Matt
>
>
>> --
>> jeremy
>>
> --
> 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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