G. D. Mahan: Solid State Physics, ed. H. Ehrenreich, F. Seitz and D. Turnbull (Academic Press, New York and London, 1974) Vol. 29, p. 75, and many references on theory therein.
P. Nozieres and C. T. De Dominicis: Phys. Rev. 178 (1969) 1097.
S. Doniach and M. Sunjic: J. Phys. C 3 (1970) 285.
L. Ley, F. R. McFeely, S. P. Kowalczyk, J. G. Jenkin and D. A. Shirley: Phys. Rev. Bll (1975) 600.
P. H. Citrin, G. K. Wertheim and Y. Bear: Phys. Rev. Letters 35 (1975) 885.