1. Reitz, J.R., Mayer, F.J.: J. Math. Phys. 41, 4572 (2000)
2. Schiff, L.I.: Quantum Mechanics, Second Edition. McGraw-Hill, New York (1955)
3. Messiah, A.: Quantum Mechanics, vol.II, Wiley, New York, (1962)
4. Barut, A.O., Bracken, A.J.: Phys. Rev D 24, 3333 (1981)
5. Slater, J.C.: Quantum Theory of Atomic Structure, pp 189–192, McGraw-Hill, New York (1960). Also Schiff, ibid, p. 283