1. See for example Parks, R.D. (ed.): Valence instabilities and related narrow band phenomena. New York: Plenum Press 1977
2. Leder, H.J., Muhlschlegel, B.: Z. Physik B29, 341 (1978); Entel, P., Leder, H.J., Grewe, N.: Z. Physik B30, 277 (1978); Entel, P., Leder, H.J. (to be published)
3. Kondo, J.: Progr. Theor. Phys. (Kyoto)28, 846 (1962)
4. de Chatel, P.F., Aarts, J., Klaasse, J.C.P.: Comm. Phys.2, 151 (1977)
5. Ghatak, S.K.: Physica (1976)