Re: [Libmesh-users] BoomerAMG/hypre with CG for elasticity

[Roy S. <roy...@ic...>]

On Mon, 3 Feb 2014, Jens Lohne Eftang wrote:

> With libmesh 0.9.3-pre, If I do use a DirichletBoundary (with a ZeroFunction 
> or ConstFunction), and then after calling system.solve() loop over my system 
> matrix entries to check symmetry, my matrix is symmetric to machine 
> precision.

What do the *constrain_elem* calls in your code look like?

Oh, wait, I'm being silly.  If you've only got constraints from
Dirichlet boundaries, then it doesn't matter whether you explicitly
enforce symmetry or not; the constraint equation "x_i = c" just has a
1 on the diagonal and no off-diagonal components.  It's only AMR
constraints that are likely to break symmetry.

> And also the linear solver would have complained about any lack of 
> symmetry (I would assume).

Not a safe assumption; testing for symmetry can be expensive.  I
recall once running into a preconditioner that would just silently
"converge" to the wrong answer if fed an asymmetric matrix.
---
Roy