1. N. N. Bogoliubov andD. V. Shirkov:Introduction to the Theory of Quantized Fields (New York, N. Y., 1959).
2. K. G. Wilson:Phys. Rev. D,10, 2445 (1974);A. M. Polyakov:Phys. Lett.,59 B, 82 (1975);J. Kogut andL. Susskind:Phys. Rev. D,11, 395 (1975).
3. See, for example,M. Wortis:Linked cluster expansion, inPhase Transitions and Critical Phenomena, Vol.3, edited byC. Domb andS. Green (London, 1974), and references quoted therein.
4. G. Wentzel:Helv. Phys. Acta,13, 269 (1940);S. Tomonaga:Prog. Theor. Phys.,2, 6 (1947);F. L. Gervais andB. Sakita:Nucl. Phys.,91 B, 301 (1975).
5. a)E. R. Caianiello andG. Scarpetta:Nuovo Cimento,22 A, 448 (1974);Lett. Nuovo Cimento,11, 283 (1974).b)W. Kainz:Lett. Nuovo Cimento,12, 217 (1975);H. G. Dosch:Nucl. Phys.,96 B, 525 (1975).