1. H. Haken, in: Halbleiterprobleme Bd. II. 1955, herausgegeben von W. Schottky, Friedr. Vieweg & Sohn, Braunschweig 1955.
2. C. G. Kuper and G. D. Whitfield, Polarons and Excitons, Scottish Universities Summer School, 1962, Oliver and Boyd, Edinburgh and London.
3. J. Appel in: Solid State Physics, edited by F. Seitz, D. Turnbull and H. Ehrenreich, Academic Press, New York 1968, vol. 21.
4. J. Devreese, Fröhlich Polarons and Electron-Phonon Interaction in Polar Crystals Summer School of the University of Antwerp, North Holland (1972).
5. I. G. Austin and N. F. Mott Adv. Phys. 18, 41 (1965).