1. J. Schwinger, Proc. Nat. Acad. Sci. 44, 956 (1958).
2. K. Symanzik in Proc. Int. School “Enrico Fermi”, R. Jost ed. Acad Press (1969).
3. K. Osterwalder, R. Schrader, Comm. Math. Phys. 31, 83 (1973).
4. M. Veltman, Invited talk presented at the International Symposium on Electron and Photon Interactions at High Energies, Bonn 27–31 August 1973.
5. C.N. Yang and R. Mills, Phys. Rev. 96, 191 (1954).