1. From the large number of papers we only cite some recent ones, where further references can be found: Benoit A la Guillaume, C; Voos, M.; Salvan, F.: Phys. Rev. B5, 3049 (1972);
2. Benoit A la Guillaume, C; Voos, M.; Salvan, F.: Phys. Rev. B7, 1723 (1973)
3. Brinkman, W.F.; Rice, T.M.: Phys. Rev. B7, 1508 (1973)
4. Combescot, M.; Nozières, J.: Phys. C. 5, 2369 (1972)
5. Vashishta, P.; Bhattachyya, P.; Singwi, K.S.: Phys. Rev. Lett. 30, 1280 (1973)