1. G. Veneziano:Nuovo Cimento,57 A, 190 (1968).
2. G. F. Chew:S-Matrix Theory of Strong Interactions (New York, 1961).
3. S. Mandelstam:Phys. Rev.,166, 1539 (1968).
4. M. Jacob:Theory of hadronic interactions, inRapporteur’s talk at the Lund Conference, 1969 (TW. 1052-CERN).
5. E. Predazzi:Recent developments in high energy physics, Indiana University, Further references are cited in this paper.