1. G. Veneziano, Nuovo Cimento 57A (1968) 190.
2. K. Bardakci and H. Ruegg, Phys. Rev. 181 (1969) 1884; C.J. Goebel and B. Sakita, Phys. Rev. Lett. 22 (1969) 256; Chan Hong-Mo and T. S. Tsun, Phys. Lett. 28B (1969) 485.
3. J. Paton and Chan Hong-Mo, Nucl. Phys. B10 (1969) 519.
4. C. Lovelace, Phys. Lett. 28B (1968) 265;
5. J. Shapiro, Phys. Rev. 179 (1969) 1345.