A REARRANGEMENT OF THE FEENBERG PERTURBATION FORMULA AND ELEMENTARY EXCITATION INTERACTIONS IN BOSE FLUIDS

The Feenberg perturbation energy for two quasiparticle excitations is rearranged to be valid in many-boson systems up to fifth order. Doubly excited states in many particle boson systems are studied using Feenb erg perturbation theory. The effective interaction energy between quas iparticles, which is described by ''ladder state diagrams'' - in Bogol iubov approximation, is obtained. It is then shown that in this case t he interaction energy depends on density (pressure) and changes its si gn when density increases if the bare two-body potential qualitatively has short range repulsion and longer range attraction.