1. M. Le Bellac andG. Plaut:Nuovo Cimento,64 A, 95 (1969);S. G. Brown:Phys. Rev. D,l, 207 (1970);J. P. Ader,M. Capdeville andPh. Salin:Nucl. Phys.,17 B, 221 (1970);H. Fraas andD. Schildknecht:Nucl. Phys.,6 B, 395 (1968);W. T. Potter andJ. D. Sullivan:Nuovo Cimento,68 A, 623 (1970).
2. A. Bialas, andK. Zalewski:Phys. Lett.,28 B, 436 (1969);D. Schildknecht:Zeits. Phys.,229, 278 (1969);P. D. Manheim andU. Maor: University of Tel Aviv preprint 69-94;J. Donohue: Phys. Rev. D,1, 1972 (1970).
3. See, for example,Dispersion Relations and the Abstract Approach to Field Theory, Edited byL. Klein (New York, 1961).
4. For a more complete discussion of this philosophy as well as a general treatment of singularity hunting in perturbation theory, sce:R. J. Eden,P. V. Landshoff,D. Olive andJ. C. Polkinghorne:The Analytic S-Matrix (Cambridge, 1966).
5. In complete contrast to the mathematical situation.