[petsc-dev] Scatter a single local value to multiple global values

[Chris Eldred]

Ok, it is apparent I will have to think about this some more. Thanks
for the help!

-Chris

On Wed, Feb 22, 2017 at 6:36 PM, Jed Brown <jed at jedbrown.org> wrote:
> Chris Eldred <chris.eldred at gmail.com> writes:
>
>> Hey Jed,
>>
>> That makes sense now, thanks! It sounds like the correct way to do things is:
>>
>> For matrices:
>> 1) for each set of coupled dofs, select 1 to be the master and modify
>> the local to global maps such that all the local indices that
>> previously pointed to the slave dofs now point to the master dof
>> -> assembly doesn't produce anything for the slave rows
>> 2) fill the rows for the slave dofs with a -1 in the master column and
>> 1 in the slave column (since we are just enforcing slave = master).
>
> It's better to not store slave dofs in the global matrix.  Just use the
> local-to-global mapping so that you get it in your local spaces.  What
> you describe makes the matrix nonsymmetric and possibly ill-conditioned
> (depends on scaling of the other dofs).
>
>> For residuals, just do 1) and then set slave dofs equal to the master dof.
>
> What you describe above with +1 and -1 requires a right-hand side of 0.



-- 
Chris Eldred
https://www.math.univ-paris13.fr/~eldred/
Postdoctoral Fellow, LAGA, University of Paris 13
PhD, Atmospheric Science, Colorado State University, 2015
DOE Computational Science Graduate Fellow (Alumni)
B.S. Applied Computational Physics, Carnegie Mellon University, 2009
chris.eldred at gmail.com