1. A. M. Dykhne and G. L. Yudin, Sov. Phys. Usp. 21, 549 (1978).
2. A. B. Migdal, Qualitative Methods in Quantum Theory (Nauka, Moscow, 1975; Benjamin, Reading, MA, 1977).
3. V. I. Matveev and E. S. Parilis, Sov. Phys. Usp. 25, 881 (1982).
4. V. I. Matveev, Phys. Part. Nucl. 26, 329 (1995).
5. I. C. Percival, in Atoms in Astrophysics, Ed. by P. G. Burke, W. Eissner, D. G. Hummer, and I. C. Percival (Plenum, New York, London, 1983), p. 87.