1. For proceedings of recent conferences on correlated electron systems see, for instance: Willis, J.O., Thompson, J.D., Guertin, R.P., Crow, J.E. (eds.): Physics of highly correlated electron systems. Amsterdam: North Holland 1990. Physica B163, (1990);
2. Baskaran, G., Ruckenstein, A.E., Tosatti, E., Yu, Lu (eds.): Strongly correlated electron systems. Singapore: World Scientific 1990. Int. J. Mod. Phys. B3 (1989)
3. Hubbard, J.: Proc. R. Soc. London Ser. A276, 238 (1963)
4. For recent reviews see: Czycholl, G.: Phys. Rep.143, 277 (1986);
5. Fulde, P., Keller, J., Zwicknagl, G.: Solid State Phys.41, 2 (1988);