1. Weber, W.: Z. Phys. B-Condensed Matter70, 323 (1988)
2. Kugel, K.I., Khomskii, D.I.: Sov. Phys. Usp.25, 231 (1982)
3. Weber, W., Shelankov, A.L., Zotos, X.: Proc. Int. Conf. ?Materials and Mechanisms of Superconductivity-High Temperature Superconductors'. Stanford, USA 1989
4. Zaanen, J., Oles, A.M., Feiner, L.F.: Proc. NATO ARW ?Magnetic Fluctuations in HighT c Superconductors' 1990
5. Zhang, F.C., Prelovsek, P.: Phys. Rev. B37, 1569 (1988); Tang, S., Hirsch, J.E.: Phys. Rev. B37, 9546 (1988)