1. I.A. Malkin and V.I. Man'ko, Zh. Eksp. Teor. Fiz.,59, 1746 (1970); Preprint FIAN No. 15 [in Russian], P.N. Lebedev Physics Institute (1971).
2. V.V. Dodonov, I.A. Malkin, and V.I. Man'ko, Phys. Lett.,39A, 377 (1972); Physica,72, 597 (1974).
3. I.A. Malkin, V.I. Man'ko, and D.A. Trifonov, Kratkie Soobshcheniya Po Fizike, No. 5, 20, 27 (1971); J. Math. Phys.,14, 576 (1973).
4. A. Holz, Lett. Nuovo Cimento,4, 1319 (1970).
5. N.A. Chernikov, Zh. Eksp. Teor. Fiz.,53, 1006 (1967).