1. A. Einstein, Verh. Dt. Phys. Ges., Bd 18, 5 318 (1916).
2. P. A. M. Dirac, The Principles of Quantum Mechanics, 3rd ed, Oxford (1947).
3. W. E. Lamb, Jr., Impact of Basic Research and Technology, ed. B. Kursunogly, A. Perlmutter, Plenum Press (1975).
4. J. P. Gordon, H. J. Zeiger, and C. H. Townes, Phys. Rev. 95, 282 (1954).
5. N. G. Basov and A.M. Prokhorov, Zh. Eksp. Teor. Fiz. (in Russian) 27, 431 (1954).