Single Scale Analysis of Many Fermion Systems. Part 1: Insulators
  We construct, using fermionic functional integrals, thermodynamic Green's functions for a weakly coupled fermion gas whose Fermi energy lies in a gap. Estimates on the Green's functions are obtained that are characteristic of the size of the gap. This prepares the way for the analysis of single scale renormalization group maps for a system of fermions at temperature zero without a gap. 
Joel Feldman, Horst Knoerrer and Eugene Trubowitz