1. T. Matsubara:Prog. Theor. Phys.,14, 351 (1955);F. J. Dyson:Phys. Rev.,102, 1217, 1230 (1956).
2. See for exampleR. P. Feynman:Statistical Mechanics, a Set of Lectures (W. A. Benjamin, Inc., New York, N. Y., Amsterdam, 1972), p. 66.
3. C. Buzano, M. Rasetti andM. Vadacchino:Prog. Theor. Phys.,68, 703, 716 (1982).
4. A. Riccardi:Lett. Nuovo Cimento,40, 147 (1984).
5. C. Domb andM. S. Green:Phase Transition and Critical Phenomena, Vol. 3 (Academic Press, London, New York, N.Y, San Francisco, Cal, 1974), Chapt. 1.