Gauge invariance of relativistic two-particle bound-state energies. Pt. 2

The gauge invariance, in quantum electrodynamics, of the bound-state energies is proven, order by order in perturbation theory, in the case of solutions of the Bethe-Salpeter equation for the spin-1/2 fermion-antifermion system. A simple algebraic, and corresponding diagrammatic, procedure is developed to deal with the problem. It is shown that subsets of energy contributions are gauge invariant, a property which can provide a useful flexibility in calculations. (orig.)