Introduction

Abstract

The notion that ground state properties of quantum many-particle systems can be characterised solely in terms of the one-particle density is not obvious. Consider for instance the ground state energy and appeal to the description of the system with the aid of Green’s function techniques (Martin and Schwinger, 1959). The ground state energy can be expressed directly in terms of one- and two-particle Green’s functions. The determination of these quantities involves, however, a set of differential equations coupling eventually all n-particle Green’s functions, with n being less than or equal to the number of particles involved. A corresponding statement applies if one frames the problem in the language of the theory of density matrices (ter Haar, 1961).