1. Fröhlich, H.: Proc. Roy. Soc. London A223, 296 (1954)
2. Sham, L.J.: “Electronic Influence on Lattice Dynamics” on the Antwerp Conference, June 18.–30. 1973
3. Confer e.g. N.J. Zabusky: Nonlinear partial difference differential equations. Ed. W. Ames. New York: Academic Press 1967
4. Risken, H., Nummedal, K.: Phys. Lett.26 A, 275 (1968);27 A, 445 (1968); Journ. Appl. Phys.39, 4662 (1968)
5. Bonifacio, R., Narducci, L.M.: Nuovo Cimento Lett.1, 671 (1969)