1. For a general review on the theory of dual-resonance models, see,e.g.,G. Veneziano:Lectures at the Erice School (1970).
2. See, for instance,M. Ademollo:Current amplitudes in dual-resonance models, talk presented at theSymposium on Meson Photo- and Electro-Production at Low and Intermediate Energies, Bonn, 21–26 September, 1970; and references contained therein.
3. D. B. Fairlie andH. B. Nielsen:Nucl. Phys.,20 B, 637 (1970).
4. B. Sakita andM. A. Virasoro:Phys. Rev. Lett.,24, 1146 (1970).
5. H. B. Nielsen andL. Susskind: CERN preprint, TH. 1230 (1970).