1. S. Fubini:Nuovo Cimento,43, 475 (1966).
2. See alsoM. O. Taha: Cambridge preprints.
3. D. Amati, R. Jengo andE. Remiddi:Nuovo Cimento,51 A, 99 (1967).
4. B. Renner:Nuovo Cimento 50 A, 605 (1967).
5. For a description of methods for analysing analytic properties of Feynman graphs see Ch.2 of ref. (6).R. J. Eden, P. V. Landshoff, D. I. Olive andJ. C. Polkinghorne:The Analytic S-Matrix (Cambridge, 1966).