1. M. A. Virasoro:Phys. Rev. D,1, 2933 (1970);S. Fubini andG. Veneziano: MIT preprint CTP No. 128 (1970), to be published inAnn. of Phys.
2. G. Veneziano:Lecture Notes at the International School of Subnuclear Physics, Elementary Processes at High Energy, Erice, 1970.
3. M. A. Virasoro:Phys. Rev.,177, 2309 (1969)
4. J. A. Shapiro: University of Maryland technical report No. 71-039, October 1970.
5. The model (1.3) is a particular case of a more general model studied byC. Sommerfield. Private communication toS. Fubini and ref. (4).