[dune-fem] periodic conforming FE

[Andreas Dedner]

Hi.
I need a periodic conforming FE discretization. I know that there have 
been different approaches
and I was wondering if someone has something lying around in some branch.

If not then my idea was to do it along the lines of the Constraints 
class, i.e., postporcessing the
matrix and the rhs after normal assembly as we do it for the dirichlet 
data. Basically I wanted to
identify the parallel nodes, setup a master/slave vector. Then the rows 
in the matrix belonging to slave(s)
are added to the row of the master and the rows for the slaves is 
replaced by a (0..0 -1 0...0 1 0...0)
leading to u_s-u_m = 0. This would require some more methods on the 
linear operator class similar to
clear row.
If somebody has some better idea I would be happy to hear about it.

Second question: for those developing grids what semantics for periodic 
boundaries did you assume (if any).
I know that this has never been defined very well in the dune interface. 
Is it
      intersection.neighbor()=true and intersection.boundary()=true and
      intersection.outside return a "ghost"?
The documentation also states that the index of that ghost is different 
from the index of the periodic inner element (ids are the same. But DG 
seems to work out of the box so I'm assuming in dune-fem its done 
differently....

Thanks for any pointers, best
Andreas