1. For a recent review of this problem see the lectures by Littlewood, P. In: Correlated Electron Systems. Proceedings of the Jerusalem Winter School of Theoretical Physics. Emery, V.J. (ed.). Singapore: World Scientific (to appear)
2. Metzner, W., Vollhardt, D.: Phys. Rev. Lett.62, 324 (1989)
3. Janis, V.: Z. Phys. B83, 227 (1991); Janis, V., Vollhardt, D.: Int. J. Mod. Phys. B6, 731 (1992)
4. Georges, A., Kotliar, G.: Phys. Rev. B45, 6479 (1992)
5. For a recent review and references, see e. g. Vollhardt, D.: In: Correlated Electron Systems. Proceedings of the Jerusalem Winter School of Theoretical Physics. Emery, V.J. (ed.). Singapore: World Scientific (to appear). Preprint RWTH/ITPC6/92