Perturbative Quantum Field Theory at Positive Temperatures: An Axiomatic   Approach
  It is shown that the perturbative expansions of the correlation functions of a relativistic quantum field theory at finite temperature are uniquely determined by the equations of motion and standard axiomatic requirements, including the KMS condition. An explicit expression as a sum over generalized Feynman graphs is derived. The canonical formalism is not used, and the derivation proceeds from the beginning in the thermodynamic limit. No doubling of fields is invoked. An unsolved problem concerning existence of these perturbative expressions is pointed out. 
O. Steinmann (Universitaet Bielefeld, Fakultaet fuer Physik, D-33501   Bielefeld, Germany)