Fermi Surface of the 2D Hubbard Model at Weak Coupling
  We calculate the interaction-induced deformation of the Fermi surface in the two-dimensional Hubbard model within second order perturbation theory. Close to half-filling, interactions enhance anisotropies of the Fermi surface, but they never modify the topology of the Fermi surface in the weak coupling regime. 
Christoph J. Halboth and Walter Metzner