1. L. de Broglie, An Introduction to the Study of Wave Mechanics (Methuen, London, 1930).
2. P. A. M. Dirac, The Quantum Theory of the Electron (Royal Society, London, 1928).
3. W. Heisenberg, Zs. Naturforsch. 9a, 292 (1954).
4. F. Gursey, Nuovo Cimento 3, 988 (1956).
5. F. Kortel, Nuovo Cimento 4, 210 (1956).