1. D. I. Blokhintsev, Collection: Philosophical Questions of Modern Physics [in Russian] (Izd. AN SSSR, Moscow, 1952), p. 358.
2. N. N. Bogolyubov and D. V. Shirkov, Introduction to the Theory of Quantum Fields [in Russian] (Gostekhteorizdat, Moscow, 1957), Section 17.
3. M. Firts, Collection: Latest Developments in Quantum Electrodynamics [Russian translation] (IL, Moscow, 1954), p. 238.
4. D. I. Blokhintsev, OIYaI Preprint [in Russian] (Dabna, 1962).
5. M. Born, cited in the book by W. Heitler, Quantum Theory of Radiation [Russian translation] (IL, Moscow, 1956).