1. J.-P. M. Lebrun:Nuovo Cimento A,99, 211 (1988);100, 477 (1988);102, 925 (1989).
2. P. A. M. Dirac:Phys. Rev.,74, 817 (1949).
3. D. Landau andE. Lifshitz:Classical Fields (Pergamon Press, London, 1959);R. Adler, M. Bazin andM. Schiffer:General relativity (McGraw Hill, New York, N.Y., 1965).
4. P. Morse andH. Feshbach:Methods of Theoretical Physics, esp. p. 30, Vol.1 (McGraw Hill, New York, N.Y., 1953).
5. I. Gradshteyn andI. Rydzhik:Tables of Integrals, Series, and Products, esp. on hyperbolic amplitude (Academic Press, New York, N.Y., 1965).