Low-temperature expansion and perturbation theory in 2D models with   unbroken symmetry: a new approach
  A new method of constructing a weak coupling expansion of two dimensional (2D) models with an unbroken continuous symmetry is developed. The method is based on an analogy with the abelian XY model, respects the Mermin-Wagner (MW) theorem and uses a link representation of the partition and correlation functions. An expansion of the free energy and of the correlation functions at small temperatures is performed and first order coefficients are calculated explicitly. They are shown to coincide with the results of the conventional perturbation theory. We discuss an applicability of our method to analysis of uniformity of the low-temperature expansion. 
O. Borisenko, V. Kushnir, A. Velytsky