1. For a review, see Adler, D.: Solid State Phys.21, 1 (1968)
2. For a recent review, see Grewe, N., Steglich, F.: Handbook on physics and chemistry of rare earths. Gscheidner, K.A. (ed.), Vol. 14. Amsterdam: North-Holland 1991
3. For a review, see Physica C185?189: Proceedings of the International Conference on Materials and Mechanisms of Superconductivity. High Temperature Superconductors III (Kanazawa, Japan 22?26.7.1991)
4. Hubbard, J.: Proc. R. Soc. London Ser. A276, 238 (1963); Gutzwiller, M.C.: Phys. Rev. Lett.10, 159 (1963); Kanamori, J.: Prog. Theor. Phys.30, 275 (1963)
5. For a recent review, see Vollhardt, D.: Correlated electron s systems. Emergy, V.J. (ed.). Singapore: World Scientific (1993, in press)