1. H. Fröhlich, Adv. Phys. 3, 325 (1954).
2. V. M. Agranovich and V. L. Ginzburg, Crystal Optics with Spatial Dispersion, and Excitons, 2nd ed. (Nauka, Moscow, 1979; Springer, New York, 1984).
3. S. I. Pekar, Investigations on Electron Theory of Ion Crystals (GITTL, Moscow, 1951; Report AEC-tr-5575, U.S. Atomic Energy Commission, 1963); L. Landau, Phys. Z. Sowjet. 3, 664 (1933).
4. D. Emin, Phys. Rev. B 48, 1369 (1993).
5. W. Heitler, The Quantum Theory of Radiation, 3rd ed. (Clarendon, Oxford, 1954; Inostrannaya Literatura, Moscow, 1956).