1. L. I. Schiff, Quantum Mechanics, McGraw-Hill, New York, 1968.
2. F. Iachello and A. Arima, The interacting boson model, Cambridge University Press, Cambridge, 1987.
3. J. Cseh, Proc. 4th Int. Wigner Symposium, Guadalajara, Mexico, 1995.
4. J. P. Elliott, Proc. Roy. Soc. A 245 (1958), 128 and 562.
5. K. Wildermuth and Th. Kanellopoulos, Nucl. Phys. 7 (1958), 150.