1. For an excellent description of the theory of dual models, see the 1970 Brandeis lectures of S. Mandelstam, edited by Deser, Grisaru and Pendleton, MIT Press, Vol. 1, and the more recent article by J.H. Schwarz in Physics Reports 8C, No 4 (September 1973).
2. Y. Nambu - Lectures at the Copenhagen Summer Symposium (1970);
3. O. Hara - Progr.Theor.Phys. 46, 1549 (1971);
4. T. Goto - Progr. Theor. Phys. 46, 156O (1971);
5. L. N. Chang and T. Mansouri - Phys.Rev. 2535 (1972);