1. Z. Fried and W.M. Frank in “Generation of Beats from a microscopic viewpoint” to appear shortly in Il Nuovo Cimento.
2. A note on non-linear effects in electrodynamics
3. Note on the Radiation Field of the Electron
4. S.S. Schweber, “An Introduction to Relativistic Quantum Field Theory” (Row, Peterson and Company, Evanston, Illinois) p. 102.