1. A. Gara, B. Durand, L. Durand and L. J. Nickisch, Phys. Rev. D 40 (1989) 843; A. Gara, B. Durand and L. Durand, Phys. Rev. D 42 (1990) 1651;
2. V.S. Fadin, V.A. Khoze, Sov. J. Nucl. Phys. 48 (1988) 309;
3. S.N. Gupta and F. Redford, Phys. Rev. D 24 (1981) 2309, D 25 (1982) 3430; S.N. Gupta, F. Redford, W.W. Repko, Phys. Rev. D 26 (1982) 3305; J. Pantaleone, S.-H.H. Tye, Y.J. Ng, Phys. Rev. D 33 (1986) 777; F. Halzen, C. Olson, M.G. Olsson and M.L. Stong, Phys. Rev. D 47 (1993) 3013;
4. W. Kummer and W. Mödritsch, Z. Phys. C 66 (1995) 225;
5. R. Barbieri and E. Remiddi, Nucl. Phys. B 141 (1978) 413;