1. See, Y.E. Pokrovsky; Phys. Status Solidi (A), 384–(1972).
2. See, T.M. Rice in Solid State Physics, edited by H. Ehrenreich, F. Seitz and D. Turnbull (Academic Press, New York, 1977 ), Vol. 32. J.C. Hensel, T.G. Phillips and G.A. Thomas, Ibid.
3. See, P. Vashishta, P.K. Kalia and K.S. Singwi in Electron-Hole Liquid edited by L.V. Keldish and D.C Jeffries (North Holland, Amsterdam, to be published.)
4. J. Lindhard, K. Dan. Vidensk Selsk, Mat-fys. Medd. 28, 8 (1954). D. Pines and D. Böhm, Phys. Rev. 85, 338 (1952); 92, 609 (1953).
5. J. Hubbard, Proc. Roy. Soc. (London) A243, 336 (1957).