On Wed, 2 Mar 2011, Boyce Griffith wrote:
> On 3/1/11 6:41 PM, Roy Stogner wrote:
>>
>> On Thu, 2 Dec 2010, Boyce Griffith wrote:
>>
>>> The patched code does the "right thing" only when
>>> asymmetric_constraint_rows=true when calling
>>> DofMap::constrain_element_{matrix,vector,matrix_and_vector}(). This is
>>> the default, but if asymmetric_constraint_rows=false, the code will
>>> silently do the "wrong thing," probably yielding bizarre results. One
>>> approach would be to keep track of whether a particular constraint row
>>> is provided by the user or not, and to require
>>> asymmetric_constraint_rows==true when such constraints are present.
>>
>> Thanks for the heads-up. I've got some FEMSystem-based codes that
>> could probably be adjusted pretty easily to test that case, which we'd
>> want to fix before committing.
>
> Let me know if you want me to think about what to do for this case, or if it
> isn't clear what goes wrong if asymmetric_constraint_rows=false.
I haven't looked at the problem too closely yet, but it might be
better to think about the math before the code.
For the inhomogeneous cases: we basically turn K*x = F, x=C*y into
C^T*K*C*y=C^T*F. That's still a symmetric matrix. It's a smaller
symmetric matrix in the case where length(y) < length(x), but we don't
actually condense the matrix, so we end up having indeterminate
hanging node entries in y. In the symmetric case we make the matrix a
diagonal for those entries, and then we fix them up later after the
linear solve with enforce_constraints_exactly. In the asymmetric case
we reuse those entries' lines with the constraint equations so that
the solver will enforce them (if only inexactly).
So now we've got x=C*y+H
Which means if we do things analogously in the symmetric case we want
C^T*K*(C*y+H)=C^T*F
C^T*K*C*y = C^T*(F-K*H)
Hmm... this isn't what your patch does, and when I think about it we
probably *don't* want to do things analogously (it's nice that in the
homogeneous case x is also just y with some entries omitted, but the
"x=C*y+H" model doesn't preserve that property). Is there any
similarly clean way to formulate what your patch is doing?
---
Roy
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