1. For a review see for example: Fulde, P.: Crystal fields (Chapter 17) in: Handbook on the physics and chemistry of rare earths (edit. by K.A. Gschneidner and Le Roy Eyring) to be published
2. Becker, K.W., Fulde, P.: In: Crystal Field Effects in Metals and Alloys (edited by A. Furrer). New York: Plenum Press 1977
3. Messiah, A.: Quantum Mechanics, Vol. II, p. 994?997. Amsterdam: North-Holland Publishing Company 1965
4. Fulde, P., Peschel, I.: Adv. in Physics21, 1 (1972)
5. Walker, E., Purwins, H.-G., Landolt, M., Hulliger, F.: J. Less-Common-Metals33, 203 (1973)