  A quantum-field approach to studying the Bose systems at finite temperatures
and in states with spontaneously broken symmetry, in particular in a superfluid
state, is proposed. A generalized model of a self-consistent field (SCF) for
spatially inhomogeneous many-particle Bose systems is used as the initial
approximation. A perturbation theory has been developed, and a diagram
technique for temperature Green's functions (GFs) has been constructed. The
Dyson's equations joining the eigenenergy and vertex functions have been
deduced.
