Explicit Formulas inDegenerateRayleigh-Schrödinger Perturbation Theory for the Energy and Wave Function, Based on a Formula of Lagrange

Abstract
The solution for the energy and wave function in degenerate Rayleigh-Schrödinger perturbation theory is obtained via a formula of Lagrange. The derivation involves manipulation of the Brillouin-Wigner formulas in a manner similar to the "repeated-partitioning" technique.