1. See, for instance, papers inQuantum Gravity: An Oxford Symposium, edited byC. J. Isham, R. Penrose andD. W. Sciama (Oxford, 1975).
2. M. A. Naimark:LInear Representations of the Lorentz Group (New York, N. Y., 1964).
3. M. A. Naimark:Normed Rings (Groningen, 1959).
4. N. I. Akhiyezer andI.M. Glazman:The Theory of Linear Operators in Hilbert Space (New York N. Y., 1961).
5. M. Carmeli:Phys. Lett. A,28, 683 (1969).