Formal Rayleigh–Schrödinger perturbation theory for both degenerate and non‐degenerate energy states

Using the Rayleigh‐Schrödinger perturbation formalism, expressions are obtained for the n th order energy and the n th order wave function. No restrictions are placed on the degeneracy of the state or the order of perturbation in which the degeneracies are resolved. By the use of suitable operators, the formulae are expressed in compact form.