Approximations in many-body systems and Heisenberg's equations

Abstract

In this paper some selfconsistent approximations are developed, especially of the kind, in which undressed Green's functions and undressed interaction-lines are used. Apart from the self-consistent description of thermodynamical properties these approximations fulfill the thermodynamical average of Heisenberg's equations of motion. As will be shown these equations can be derived from an invariance-property of the thermodynamic potential.