1. P. A. M. Dirac, Europhysics News 8, 1 (1977) and Proceedings of the Conference on Particle Physics, Budapest 1977.
2. F. Rohrlich, Chapter 13 of this volume.
3. A. O. Barut and J. P. Crawford, Phys. Lett. B. 82, 233 (1979).
4. A. O. Barut, Phys. Lett. B
73, 310 (1978).
5. A. O. Barut, Lepton mass formula, Phys. Rev. Len. 42, 1251 (1979).