1. V.T. Agekyan, Phys. Stat. Sol. (a) 43, 11 (1977).
2. R.M. Martin and L.M. Falicov, in “Light Scattering in Solids”, ed. M. Cardona, Springer Verlag (1975), vol. 8, p. 79.
3. A summary of these properties is found in the Third International Conference on Light Scattering in Solids, Campinas, Brazil11, ed. M. Balkanski, R.C.C. Leite and S.P.S. Porto, Flammarion, Paris (1976), p. 9, 50.
4. J.L. Birman, Sol. Stat. Comm. L3, 1189 (1973).
5. J.L. Birman, Phys. Rev. B9, 4518 (1974).