Thermal Operator Representation of Finite Temperature Graphs
  Using the mixed space representation (t,p) in the context of scalar field theories, we prove in a simple manner that the Feynman graphs at finite temperature are related to the corresponding zero temperature diagrams through a simple thermal operator, both in the imaginary time as well as in the real time formalisms. This result is generalized to the case when there is a nontrivial chemical potential present. Several interesting properties of the thermal operator are also discussed. 
Fernando T. Brandt, Ashok Das, Olivier Espinosa, Josif Frenkel,   Silvana Perez