1. J.G. Bednorz and K.A. Müller Z. Phys. B64:189(1986).
2. T. Holstein, Ann. Phys. (N.Y.) 8:325(1959).
3. A.S. Alexandrov and A.B. Krebs Soy. Phys. Usp. 35:345(1992).
4. H. De Raedt and A. Lagendijk Phys. Rev. B30:1671(1984).
5. J.E. Hirsch and E. Fradkin Phys. Rev. Lett. 49:402(1982);