1. H. Poincare, “Dynamics of Electrons,” Compt. Rend., 140, 1504 (1905);
2. H. Poincare, “Surla dynamique de l’electron,” Circolo Mat. Palermo Rend., 21, 129 (1906).
3. An interesting problem in this connection was broached recently by M. F. M. Osborne, “Quantum Theory Restrictions on the General Theory of Relativity,” Bull. Am. Phys. Soc., 24, 2 (Berkeley Meeting), Paper A-3 (1949).
4. In classical theory, this observation is due to F. Klein’s school. Cf. also F. Engel, “Uber die zehn allgemeinen Integrale der klassischen Mechanik,” Nachr. Kgl. Ges. Wiss. Göttingen, p. 270 (1916);
5. G. Hamel, “Die Lagrange-Eulerschen Gleichungen der Mechanik,” Z. Math. Phys., 50, 1 (1904)