1. J. Nambu, “Quark model and the factorization of the Veneziano amplitude,” in: Symmetries and Quark Models (R. Chand, ed.), Gordon and Breach, New York (1970), pp. 269–278.
2. M. B. Green, J. H. Schwarz, and E. Witten, Superstring Theory, Cambridge Univ. Press, Cambridge (1987).
3. C. Lovelace, Phys. Lett. B, 28, 264–268 (1968)
4. D. V. Shirkov, Sov. Phys. Usp., 13, 599–607 (1970).
5. A. V. Anisovich, V. V. Anisovich, and A. V. Sarantsev, Phys. Rev. D, 62, 051502 (2000).