Quasi-degenerate many-body Rayleigh-Schroedinger perturbation theory

Based on the Hugenholtz diagrammatic technique (resolvent operator and factorization theorem with complex convolution) a quasi-degenerate many-body Rayleigh-Schroedinger perturbation theory is derived. The derivation is based on an algebraic similarity between the formal generalized characteristic problem and its many-body version. The diagrammatic expressions for the perturbed state vectors and the mean values of an observable are obtained. The relationship between the method and the folded-diagram approach is discussed. (author)