1. Ya. I. Frenkel’, Zh. Eksp. Teor. Fiz. 6, 647 (1936); Usp. Fiz. Nauk 93, 408 (1967).
2. A. S. Davydov, Izv. Akad. Nauk SSSR, Ser. Fiz. 12, 608 (1948).
3. S. I. Pekar, Selected Works (Naukova. Dumka, Kiev, 1988) [in Russian].
4. E. F. Gross, Usp. Fiz. Nauk 63, 576 (1957).
5. E. I. Rashba, in Excitons, Ed. by E. I. Rashba and M. D. Sturdge (North-Holland, Amsterdam, 1982), Chap.13.