Quasi-degenerate many-body Rayleigh-Schrödinger perturbation theory

An "Effective Hamiltonian" for quasi-degenerate many-body perturbation theory

Rayleigh-Schrõdinger perturbation theory revisited and extended

Time-Independent Rayleigh-Schrödinger Perturbation Theory

On a Modification of the Rayleigh-Schrödinger Perturbation Theory

Quasi-Degenerate Perturbation Theory

Perturbation theory for the degenerate case in the many-body problem