1. H. A. Bethe and E. E. Salpeter, Quantum Mechanics of One and Two Electron Atoms ( Springer-Verlag, Berlin, 1957 ).
2. J. C. Slater, Quantum Theory of Atomic Structure (McGraw-Hill, New York, 1960 ), Vol. II.
3. See definition and explanation below.
4. K. Rajnak and B. G. Wybourne, Phys. Rev. 132, 280 (1963).
5. G. Racah and J. Stein, Phys. Rev. 156, 58 (1967).