1. Wheeler, J. A., 1976, Studies in Mathematical Physics: Essays in Honor of Valentine Bargmann, pp 351–422, edited by E. H. Lieb, B. Simon and A. S. Wightman. Princeton University Press, Princeton
2. Yngve, S., 1986, Phys Rev A33, 96–104
3. Yngve, S., 1988, J Math Phys 29, 931–936
4. Larsson, K., and Fröman, P. O., 1994, Phil Trans R Soc Lond A347, 23–35
5. Karlsson, F., and Jedrzejek, C., 1987, J Chem Phys 86, 3532–3538