Spectral Representation at Finite Temperature
  This is a short review on the thermal, spectral representation in the real-time version of the finite temperature quantum field theory. After presenting a clear derivation of the spectral representation, we discuss the properties of its spectral function. Two applications of this representation are then considered. One is the solution of the Dyson equation for the thermal propagator. The other is the formulation of the QCD sum rules at finite temperature. 
Sourav Sarkar, B. K. Patra, V. J. Menon, and S. Mallik