1. See Z.B.Goldschmidt: In Handbook on the Physics and Chemistry of Rare Earths, Vol. 1, ed. by K.A.Gschneidner, Jr., L.Eyring (North-Holland, Amsterdam 1978) p. 1
2. C.M.Varma: Rev. Mod. Phys. 48, 219 (1976)
3. Proc. of Intern. Conf. on Magnetic Semiconductors, Jülich, FR. Germany (1975), ed. by W.Zinn (North-Holland, Amsterdam 1976)
4. Proc. of Intern. Conf. on Magnetic Semiconductors, Montpellier, France (1979), J. Physique 41, C5 (1980)
5. Valence Instabilities and Related Narrow Band Phenomena, ed. by R.D.Parks (Plenum Press, New York 1977)