Projection operator approach to time-independent perturbation theory in quantum mechanics

The time-independent perturbation theory in quantum mechanics is formulated using projection operator techniques. The determination of the perturbed eigenvalue can be decoupled from that of the perturbed eigenstate. Both the Brillouin-Wigner theory and the Rayleigh-Schrodinger theory come out straightforwardly. Both degenerate and nondegenerate cases can be treated in a unified way for arbitrarily high order perturbations. (C) 2000 American Association of Physics Teachers.