Boundary Conditions and Correlations in Path Integrals for Quantum Field   Theory of Thermal and Non-Equilibrium States
  For thermal equilibrium systems it is shown, how the Kubo-Martin-Schwinger boundary condition may be used to factorize the generating functional of Green functions at least on the level of the full two-point function. Genuine non-equilibrium system exhibit correlations that one may also incorporate in the path integral. One one hand this provides a natural tool for a perturbative expansion including these correlations. On the other hand it allows to prove that in general non-equilibrium systems the generating functional does not factorize. 
P.A.Henning and R.Fauser