Small energy denominators in interacting quantum systems: Bound states

Abstract
A perturbative method is developed for calculating bound states of interacting quantum systems, which is based on an analysis of terms with small energy denominators. An iterative scheme is formulated in a systematic manner which eliminates small energy denominators completely. The method is applied to the φ4 model of interacting bosons. The zeroth order solution of the equation of motion differs significantly from the usual free solution, and satisfies a different equation, the determining or bifurcation equation. The additional information contained in this zeroth approximation is used to calculate properties of bound states.