1. S. K. Joshi andA. K. Rajagopal: inSolid State Physics, Vol. 22 (Academic Press, New York, N. Y., 1968), p. 159.
2. V. Heine: inSolid State Physics, Vol. 24 (Academic Press, New York, N.Y., 1971), p. 1.
3. G. K. Horton andA. A. Maradudin: inDynamical Properties of Solids (North Holland Publ. Co., Amsterdam, 1980).
4. W. A. Harrison:Phys. Rev.,181, 1036 (1969).
5. J. A. Moriarty:Phys. Rev. B,1, 1363 (1970);5, 2066 (1972).