A Re-examination of the Path Ordered Approach to Real Time Thermal Field   Theory
  We argue that the asymptotic condition should not be applied in the derivation of the real time formalism of thermal field theory. It is shown that, contrary to popular belief, the generating functional of time ordered Green functions does not factorise. When no asymptotic condition is applied to the real time formalism, we find that the normal two component Feynman rules arises naturally. In addition, the extra Feynman rule that is applied when calculating vacuum diagrams is simply derived. We also clear up any doubts regarding the equivalence of the real and imaginary time formalisms. Finally, we consider these results in the case of the new real time contour. 
T.S.Evans and A.C.Pearson