1. E. P. Wigner:Ann. of Math.,40, 149 (1939).
2. I. M. Gelfand, R. A. Minlos andZ. Ya. Shapiro:Representations of the Rotation and Lorentz Groups and Their Applications (Oxford and New York, N. Y., 1963);M. A. Naimark:Linear Representations of the Lorentz Group (London, 1964). These are two standard references on this subject. It is not our intention to give a complete list of the relevant literature which would probably fill several dozens of pages.
3. R. Fricke andF. Klein:Vorlesungen über die Theorie der elliptischen Modulfunktionen, Vol.1 (Leipzig, 1890); Vol.2 (Leipzig, 1892).Vorlesungen über die Theorie der automorphen Funktionen, Vol.1 (Leipzig, 1897); Vol.2, part 1 (Leipzig, 1901); part 2 (Leipzig, 1912). These books have been reprinted by the Johnson Reprint Corporation (New York, N.Y., 1966).
4. L. R. Ford:Automorphic Functions (New York, N. Y., 1972).
5. J. Lehner:A Short Course in Automorphic Functions (New York, N. Y., 1966).