Towards a Direct Method for the Analyticity of the Pressure for Certain   Classical Unbounded Spin Systems
  The aim of this paper is to study direct methods for the analyticity of the pressure for certain classical unbounded spin models. We provide a representation in terms of the Witten Laplacian on one-forms of the nth-derivative of the pressure as function of some order parameter t . The technique involves the formula for the covariance of two functions introduced by B. Helffer and J. Sjostrand. 
Assane Lo