title: Interacting Bose gas: mean field and fluctuations revisited.
abstract: We present the description of thermodynamic properties of a Bose gas by considering the hierarchy of equations satisfied by imaginary-time Green functions. In the case of an ideal gas the appearance of the off-diagonal long-range order (presence of condensate) is derived in a particularly simple way. The simplicity of the analysis persists at the level of the mean-field theory. Using Kac's scaling of the repulsive binary potential we then determine asymptotic corrections to the mean-field theory for thermodynamic states far from the region of the mean-field condensation. Our predictions are in accordance with rigorous results obtained previously, whereas the Hartree-Fock approximation is shown to neglect a whole class of equally important contributions.
authors.affiliation: Laboratoire de Physique, ENS Lyon and CNRS, 46 allée d'Italie, 69364 Lyon Cedex 07, France.
authors.forename: A
authors.initials: A
authors.lastname: Alastuey
authors.forename: J
authors.initials: J
authors.lastname: Piasecki