1. The following are reviews providing more references:V. Alessandrini, D. Amati, M. Le Bellac andD. Olive:Phys. Rep.,1 C, No. 6 (1971). We use the convention of the above review and particularly the convention for metric $$g^{\mu v} $$ =diag (1, −1, −1, −1).V. Alessandrini andD. Amati:Dual models: Their group theoretic structure, CERN preprint TH 1425 (1971).
2. S. Mandelstam:Phys. Rev. D,1, 1720, 1734 (1970).
3. P. Ramond:Phys. Rev. D,3, 2415 (1971).
4. A. Neveu andJ. Schwarz:Phys. Rev. D,4, 1109 (1971).
5. C. B. Thorn:Phys. Rev. D,4, 1112 (1971).