1. For general discussions see, for instance: M. A. Naimark,Linear Representations of the Lorentz Group (Macmillan, New York, 1964); I. M. Gel'fand, R. A. Minlos, and Z. Ya. Shapiro,Representations of the Rotation and Lorentz Groups and Their Applications (Macmillan, New York, 1963).
2. E. Majorana,Nuovo Cimento 9, 335 (1932).
3. P. A. M. Dirac,Proc. R. Soc. London A 183, 284 (1945).
4. Harish-Chandra,Proc. R. Soc. London A 189, 372 (1947).
5. I. M. Gel'fand and M. A. Naimark,Izv. Akad. Nauk SSSR 11, 411 (1947).