  Relativistic formalism of Green's functions is dicussed in QCD and QED,where
the relativistic Green's functions are constructed using the Schwinger proper
time formalism and the Fock-Feynman-Schwinger method.As a result a simple and
exact method is found for the relativistic systems,where the interaction can be
written in a time-independent form.In this case one can write the relativistic
Green's function as a one-dimensional integral of the corresponding
nonrelativistic Green's function.The explicit example for the problem of a
charge in the constant magnetic field is discussed in detail,and the exact
agreement with the Schwinger relativistic form is demonstrated. A similar
analysis is performed in the relativistiv Coulomb probem,supporting the
accuracy of the proposed relativistic formalism.
