PERTURBATION THEORY QUANTUM MECHANICS COUPLING DIPOLE MOMENT ECUACIONES EQUATIONS FORMULA DE RAYLEIGH-SCHROEDINGER HAMILTONIAN HYPERFINE STRUCTURE MAGNETIZATION NUCLEAR MAGNETOSTATIC NONLINEAR PROBLEMS PROBLEMAS NO LINEALES SERIES EXPANSION TEORIA DE LAS PERTURBACIONES The contact hyperfine interaction: An ill-defined problem The derivation of effective hamiltonians The generating equations of Rayleigh—Schrödinger perturbation theory