1. Cf.,e.g.,J. D. Bjorken: inProceedings of the 1971 International Symposium on Electron and Photon Interactions (Ithaca, N. Y., 1971).
2. G. Altarelli andL. Maiani: preprint ISS-72-8, Rome (1972).
3. J. D. Bjorken andS. J. Brodsky:Phys. Rev. D,1, 1416 (1970).
4. E. Fermi:Progr. Theor. Phys.,1, 570 (1950).
5. R. Hagedorn:Suppl. Nuovo Cimento,3, 147 (1965).