  The Hubbard model on the honeycomb lattice describes charge carriers in
graphene with short range interactions. While the interaction modifies several
physical quantities, like the value of the Fermi velocity or the wave function
renormalization, the a.c. conductivity has a universal value independent of the
microscopic details of the model: there are no interaction corrections,
provided that the interaction is weak enough and that the system is at half
filling. We give a rigorous proof of this fact, based on exact Ward Identities
and on constructive Renormalization Group methods.
