Analytic continuation of thermalN-point functions from imaginary to real energies

Thermal n-point Green functions in the framework of quantum field theory at finite temperature are considered. We reanalyze how analytic continuations from imaginary to real energies relate these functions originally defined in the imaginary-time formalism to retarded and advanced realtime ones. We described a new and rather simple method which is valid to all orders of perturbation theory and which has the further advantage that it is independent of approximations often applied in actual finite-order calculations.