1. V. Bargmann:Rev. Mod. Phys.,21, 488 (1949).
2. S. T. Ma:Phys. Rev.,69, 668 (1946);71, 195 (1947);D. ter Haar:Physica 12, 509 (1946).
3. T. F. Jordan: private communication.
4. Compare:R. Jost:Helv. Phys. Acta,20, 256 (1947).
5. See, for exampleE. L. Ince:Ordinary Differential Equations (New York, 1956).