1. I.I. Sobelman, Theory of Atomic Spectra, Alpha Science International Ltd., 2006 (Chapter 2).
2. Triplet23 Sstates in four-electron Be-like ions: Bound-state properties and hyperfine-structure splitting
3. Excited states of beryllium atom from explicitly correlated wave functions
4. C.S. Beals, Spectrophotometric studies of Wolf-Rayet stars and Novae, Publ. Obs. Victoria 6, # 9 (1934).
5. C.W. Allen, in: A.N. Cox (Ed.), Astrophysical Quantaties, 4th ed., Spinger–Verlag, New York-Berlin-Heidelberg, 2000.