1. Anderson, P.W.: Phys. Rev.124, 41 (1961)
2. For a review see: Gruner, G., Zawadowski, A.: In: ?Progress in low temperature physics?. Brewer, D.F. (ed.), Vol. VIIB. Amsterdam: North-Holland Publ. Co. 1978
3. For a review see: Grewe, N., Leder, H.J., Entel, P.: In: ?Festk�rperprobleme XX?. Treusch, J. (ed.). Braunschweig: Vieweg 1980; Lawrence, J.M., Riseborough, P.S., Parks, R.D.: Rep. Prog. Phys.44, 1 (1981)
4. Hewson, A.C.: J. Phys. C10, 4973 (1977); see also Ref. [8]
5. The position of zero on the ionic energy scale is arbitrary. Only energy differences like ?E ? =E 1? ?E o for one electron excitations can be compared with those of excitations in the band, counted from the Fermi level. We chooseE 0=0 and therefore ?E ?=E 1???E(B=0)