1. Wigner, E. P.: Physics Today17, 34 (1964).
2. Landau, L. D., andE. M. Lifshitz: Field Theory. § 2.2. London: Pergamon Press 1959.
3. —— —— Classical mechanics. §§ 3, 4. London: Pergamon Press 1958.
4. Bargmann, V.: Ann. Math.59, 1 (1954).
5. Landau, L. D., andE. M. Lifshitz: Quantum mechanics. § 15. London: Pergamon Press 1965.