  In this paper, we give a simple diagrammatic identification of the unique
combination of the causal n-point vertex functions in the real time formalism
that would coincide with the corresponding functions obtained in the imaginary
time formalism. Furthermore, we give a simple calculational method for
evaluating the temperature dependent parts of the retarded vertex functions, to
one loop, by identifying them with the forward scattering amplitudes of
on-shell thermal particles. As an application of the method, we calculate and
show that the temperature dependent parts of all the retarded functions vanish
at one loop order for 1+1 dimensional massless QED. We further point out that,
in this model, in fact, the temperature dependent parts of all the retarded
vertex functions vanish to all orders in perturbation theory.
