See the proceedings of an international conference on the subject, ’Valence Instability and Related Narrow-Band Phenomena’ ed. R. D. Parks (Plenum, New York, 1977) and references therein.
C. M. Varma and Y. Yafet: Phys Rev. B13 (1976) 2950.
H. R. Krishna-murthy, K. G. Wilson and J. W. Wilkins: p. 177 in ref. 1.