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2. H. A. Jahn and E. Teller, Proc. R. Soc. London, Ser. A 161, 220 (1937).
3. M. A. El’yashevich, Spectra of Rare Earths (Gostekhteorizdat, Moscow, 1953) [in Russian].
4. B. G. Wybourne, Spectroscopic Properties of Rare Earths (Wiley and Sons, New York, 1965).
5. A. P. Yutsis and A. Yu. Savukinas, Mathematical Principles of Atomic Theory (Mintis, Vilnius, 1973) [in Russian].