1. W. Thirring:Ann. of Phys.,3, 91 (1958);K. Johnson:Nuovo Cimento,20, 773 (1961).
2. C. G. Callan jr.,S. Coleman andR. Jackiw:Am. Journ. Phys.,59, 42 (1970).
3. See for example:G. Veneziano:Progress in the theory and application of dual models, MIT preprint (April 1971);
4. V. Alessandrini andD. Amati:Proc. S.I.F., Course LIV (to be published), CERN preprint TH. 1925 (1971);
5. E. Del Giudice, P. Di Vecchia andS. Fubini:General properties of dual-resonance models, MIT preprint (June 1971), to appear inAnn. of Phys., and references quoted therein.