1. L. Landau and E. M. Lifshitz, Mechanics (Pergammon, Oxford, 1968), p. 151ff.
2. A. Sommerfeld, Atomic Structure and Spectral Lines (Methuen, London, 1923), p. 118ff.
3. M. Born, Mechanics of the Atom (G. Bell, London, 1927), p. 265ff.
4. On higher symmetries in quantum mechanics
5. A systematic search for nonrelativistic systems with dynamical symmetries