Sorry to bother you about periodic bounds again.
Do I understand it correctly that you implement the periodicity vectors as additional dofs that you add to the equation_systems as
Does this not give you n_elem additional dof? Whereas you actually need dim*ndof number of dof.
(I was a bit puzzled by this solution, I myself just added a neighbourless element of which I use the DOF, but I might be missing the point)
Another thing is, once you've added these dof, and used them in constraint rows related to other dofs (something like u_left + u_periodic = u_right),
do you supply any Ke referring to these dofs? As they are not (necessarily) constrained themselves,
so something must be supplied in order to avoid nonzero diagonal terms.
Of course, in case of fixed periodicity vectors in all directions one can add penalties to Ke and Fe, but I did not find a proper solution for the
case in which u_periodic is left free.
Suggestions on that topic would be very much appreciated.