1. R. London, Amer. J. Phys., 27, 649 (1959).
2. L. D. Landau and E. M. Lifshitz, Quantum Mechanics: Non-Relativistic Theory, Pergamon Press, London (1958).
3. V. V. Skobelev, Russ. Phys. J., 2017 60, No. 2, 309 (2017).
4. A. A. Sokolov, Yu. M. Loskutov, and I. M. Ternov, Quantum Mechanics [in Russian], Uchpedgiz, Moscow (1962).
5. E. A. Hylleraas, Z. Phys., 63, 291 (1930).