1. For some reviews, see: V. Alessandrini, D. Amati, M. LeBellac and D.I. Olive, Physics Reports 1C:170 (1971)
2. P. Frampton, Dual Resonance Models, Benjamin, Reading (1974)
3. S. Mandelstam, Strutural Analysis of Collision Amplitudes, Tran Than Van, ed., Les Houches, (June 1975), p. 593
4. C. Rebbi, Physics Reports 12C:1 (1974)
5. J. Scherk, Rev. Mod. Phys. 47:123 (1975)