1. W. Greiner, J. Reinhardt, Quantum Electrodynamics, 4th edn. (Springer Verlag, Berlin, 2009).
2. A. Akhiezer, V.B. Berestetskii, Quantum Electrodynamics (4th ed., Science, Moscow, (1981)) [in Russian]
3. V.B. Berestetskii, E.M. Lifshitz, L.P. Pitaevskii, Relativistic Quantum Theory (Pergamon Press, Oxford, 1971).
4. E.A. Uehling, Phys. Rev. 48, 55 (1935)
5. These units are also called the natural units (see, e.g., F. Mandl and G. Shaw, Quantum Field Theory (John Willey and Sons Ltd., New York, (1984))