Power Series Representations for Complex Bosonic Effective Actions. III.   Substitution and Fixed Point Equations
  We have previously developed a polymer-like expansion that applies when the (effective) action in a functional integral is an analytic function of the fields being integrated. Here, we develop methods to aid the application of this technique when the method of steepest descent is used to analyze the functional integral. We develop a version of the Banach fixed point theorem that can be used to construct and control the critical fields, as analytic functions of external fields, and substitution formulae to control the change in norms that occurs when one replaces the integration fields by the sum of the critical fields and the fluctuation fields. 
Tadeusz Balaban, Joel Feldman, Horst Kn\"orrer, Eugene Trubowitz
math-ph math.MP