1. E. P. Wigner, Ann. Math. 40, 149 (1939).
2. D. Han, Y. S. Kim, and D. Son, Phys. Rev. D 25, 461 (1982).
3. The fact that the compact O(2) group is only a subgroup of the full noncompact E(2) group was pointed out by Gottlieb. See H. P. W. Gottlieb, Proc. R. Soc. London A368. 429 (1979).
4. S. Weinberg, Phys. Rev. H4, B882 (1964); 135, B1049 (1964).
5. E. P. Wigner, Group Theory and Atomic Spectra, translated by J. J. Griffin (Academic, New York, 1959).