1. L. Van Hove, Phys. Rev.,89, 1189 (1953).
2. B. G. Lazarev, L. S. Lazareva, V. I. Makarov and T. A. Ignat'eva, Zh. Éksp. Teor. Fiz.,48, 1065 (1965).
3. V. I. Makarov and V. G. Bar'yakhtar, Zh. Éksp. Teor. Fiz.,48, 1717 (1965).
4. V. G. Bar'yakhtar, V. V. Gann, V. I. Makarov, and T. A. Ignat'eva, Zh. Éksp. Teor. Fiz.,62, 1118 (1972).
5. M. E. Palistrant and M. K. Kolpazhiu, Phys. Lett.,A41, 123 (1972), in: Quantum Theory of Many-Particle Systems [in Russian], Shtiintsa, Kishenev (1973).