1. Schwinger, J.:Proc. Nat. Acad. Sci. USA 46 (1960), 1401.
2. Faddeev, L. and Volkov, A.:Phys. Lett. B 315 (1993), 311.
3. Faddeev, L.: Lectures delivered at the International School of Physics ‘Enrico Fermi’ Varenna, Italy, 1994, hep-th/9408041.
4. Connes, A.:Non-commutative Geometry: Academic Press, New York, 1994.
5. Polyakov, A.:Gauge Fields and Strings, Ellis Harwood, Chichester, 1991.