Chain of equations for two-time temperature-dependent Green's functions

A study is made of the chain of equations for the re arded-advanced temperature-dependent Green's functions in the general case of a normal Fermi system with a central pair interaction. It is fotmd to be convenient to introduce a representation for the “higher” Green's functions in terms of the so-called “regular” parts of the functions and the corresponding mean values of lower order and set up a system of coupled integral equations for the “regular” parts of the Green's functions. These equations enable one to establish directly which terms of the system are the most important for a given type of interaction. Specific examples considered are a system with a Coulomb interaction and a Fermi gas with short-range repulsive forces between the particles.