Kosterlitz-Thouless Phase Transition In The Two Dimensional Linear Sigma   Model
  We investigate the O(N) symmetric linear $\sigma$-model in two dimensions by means of an exact nonperturbative evolution equation. The perturbative infrared divergences are absent in this formulation. We use a simple approximative solution of the flow equation which corresponds to a derivative expansion for the effective action. For N=2 this gives a good picture of the Kosterlitz-Thouless phase transition. 
M. Gr"ater and C. Wetterich