1. G. H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals (Wiley, New York, 1968).
2. B. R. Judd, Operator Techniques in Atomic Spectroscopy (McCraw-Hill, New York, 1963).
3. B. G. Wybourne, Spectroscopic Properties of Rare Earths (Wiley, New York, 1965).
4. B. Z. Malkin, Z. I. Ivanenko, and I. V. Aizenberg, Fiz. Tverd. Tela 12, 1874 (1970).
5. B. Z. Malkin, Quantum Theory of Paramagnetism (Kazanskii Gos. Univer., Kazan, 2006) [in Russian].