1. L. D. Landau and E. M. Lifshits, Quantum Mechanics [in Russian], Nauka, Moscow (1974).
2. A. S. Davydov, Quantum Mechanics [in Russian], Nauka, Moscow (1973).
3. Quantum Crystals [Russian translation], Mir, Moscow (1975);
4. M. I. Kaganov and I. M. Lifshits, Quasi-Particles [in Russian], Nauka, Moscow (1976).
5. I. I. Sobelman and I. V. Tyutin, Usp. Fiz. Nauk,79, 595 (1963).