On the description of quasiparticles by Green's functions

A description of statistical systems based on one-particle Green's functions is proposed. A propagator formalism is constructed without the use of field operators on the basis of some axioms for the Green's functions. These axioms are used to derive a Principle of minimality for the mean energy. The mass operator, which takes into account all manyparticle interactions, is given by a closed system of equations, in which the effective unrenormalized interaction plays a fundamental role. The unrenormalized interaction is determined either experimentally or from the variational principle.