1. We adopt the same notation as J. D. Bjorken, S. D. Drell: Relativistic Quantum Mechanics(McGraw Hill, New York 1964).
2. See H. Goldstein: Classical Mechanics, 2nd ed. (Addison- Wesley, Reading, MA 1980)
3. W. Greiner: Theoretische Physik II: Mechanik II(Hairy Deutsch, Frankfurt a.m. 1989).
4. See J. D. Jackson: Classical Electrodynamics, 2nd ed. (Wiley, New York 1975).
5. See Example 1.3 and, for a detailed discussion, J.D. Jackson: Classical Electrodynamics, 2nd ed. (Wiley, New York 1975)