Canonical solution of a system of long-range interacting rotators on a   lattice
  The canonical partition function of a system of rotators (classical X-Y spins) on a lattice, coupled by terms decaying as the inverse of their distance to the power alpha, is analytically computed. It is also shown how to compute a rescaling function that allows to reduce the model, for any d-dimensional lattice and for any alpha<d, to the mean field (alpha=0) model. 
Alessandro Campa, Andrea Giansanti and Daniele Moroni