  We consider thermal $n$-point Green functions in the framework of quantum
field theory at finite temperature. We show how analytic continuations from
imaginary to real energies relate these functions originally defined in the
imaginary-time formalism to retarded and advanced real-time ones. The described
method is valid to all orders of perturbation theory. It has the further
advantage that it is independent of approximations often applied in actual
finite-order calculations.
