1. H. A. Bethe and E. E. Salpeter, Quantum Mechanics of One-and Two-Electron Systems, Handbuch der Physik, Vol. 35/1, Springer, Berlin (1957), p. 88.
2. L. D. Landau and E. M. Lifshitz, Quantum Mechanics (Nonrelativistic Theory), 3rd ed., Pergamon, New York (1977).
3. B. M. Smirnov, Physics of Atoms and Ions [in Russian], Énergoatomizdat, Moscow (1986).
4. V. S. Lisitsa, Usp. Fiz. Nauk 153, 379 (1987) [Sov. Phys. Usp. 30, 927 (1987)].
5. V. D. Mur and V. S. Popov, Laser Phys. 3, 462 (1993).