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 Feenberg perturbation theory. The effective interaction energy between quasiparticles, which is described by "ladder state diagrams" - in Bogoliubov approximation, is obtained. It is then shown that in this case the interaction energy depends on density (pressure) and changes its sign when density increases if the bare two-body potential qualitatively has short range repulsion and longer range attraction.