2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/46436In this paper the problem of the gauge in a bound state calculation is discussed. In particular, in order to verify the gauge invariance in the energy levels expansion, some set of gauge invariant contributions are given.9 pages, Latex, 8 figures. Talk given at the NATO-ASI, Electron Theory and Quantum Electrodynamics, Edirne 8-16 September 1994High Energy Physics - Phenomenologytext