1. V. B. Berestetskii, E. M. Lifshitz, and L. P. Pitaevskii, Course of Theoretical Physics, Vol. 4: Quantum Electrodynamics (Nauka, Moscow, 1989
2. Butterworth-Heinemann, Oxford, 2012), p. 240.
3. V. Weisskopf and E. Wigner, Z. Phys. 63, 54 (1930); Z. Phys. 65, 18 (1930).
4. G. S. Wu, Y. K. Lee, N. Benczer-Koller, et al., Phys. Rev. Lett. 5, 432 (1960).
5. A. S. Davydov, Quantum Mechanics, International Series in Natural Philosophy (Pergamon, New York, 1965), p. 309.