Technical notes on a 2-d lattice O(N) model problem
  This paper provides a technical companion to M. Aguado and E. Seiler, hep-lat/0406041, in which the fate of perturbation theory in the thermodynamic limit is discussed for the O(N) model on a 2d lattice and different boundary conditions. The techniques used to compute perturbative coefficients are explained, and results for all boundary conditions considered reviewed in detail. 
Miguel Aguado