Functional methods in thermofield dynamics: A real-time perturbation theory for quantum statistical mechanics

A generating functional is constructed for real-time Green functions in quantum statistical mechanics in the context of thermofield dynamics. The KMS condition is taken as an axiom which together with field equations fixes the generating functional for causal Green functions in an interaction quantum field theory. This leads to Feynman rules for diagrammatic real-time perturbation theory. (orig.)