1. V. I. Mikerov, Candidate’s Dissertation in Mathematics and Physics, Inst. of Physics, USSR Acad. Sci., Moscow, pp. 105–108 (1977)
2. K. Tanaka, Phys. Rev. A 25, 385 (1982)
3. R. Neutze, G.E. Stedman, Phys. Rev. A 58, 82 (1998)
4. F.V. Kowalski, Phys. Lett. A 182, 335 (1993)
5. V.G. Nosov, A.I. Frank, Phys. At. Nucl. 61, 613 (1998)