1. This function was first introduced byY. Nambu: University of Chicago preprint, EFI69-64 (1969). See alsoL. Susskind: Yeshiva University preprints (1969).
2. For a comprehensive summary of the subject, seeH. M. Chan: CERN preprint, TH-1057 (1969).
3. M. A. Virasoro: University of Wisconsin preprint (1969).
4. A similar expression has been obtained byJ. M. Combes andJ. Soffer: CERN preprint, TH-1219 (1970).