1. S. Adler, Phys. Rev. 177, 2426 (1969), J. Bell and R. Jackiw, Nuovo Cim. 60 A, 47 (1969), S. Adler and W.A. Bardeen, Phys. Rev. 182, 1517 (1969).
2. S. Okubo, Phys. Lett. 5, 165 (1963), G. Zweig, CERN preprint TH412 (1964), J. Iizuka, Progr. Theor. Phys. Suppl. 37-38 (1966) 21.
3. R. Crewther, Phys. Lett. 70 b, 349 (1977), Riv. Nuov. Cim. 2, 63 (1979), Acta. Phys. Austr. Suppl XIX, 47 (1978).
4. E. Witten, Nucl. Phys. B 156, 269 (1979), G. Veneziano Nucl. Phys. B 159, 213 (1979) and Phys. Lett 95 B, 90 (1980).
5. L. Giusti, G.C. Rossi, M. Testa, and G. Veneziano, Nucl. Phys. B 628, 234 (2002) and references quoted therein.