1. F. Guerra and L.M. Morato, Phys. Rev. D27, 1774 (1983).
2. E. Nelson, “Quantum Fluctuations” (Princeton University, prss, Princeton, N.J. 1984).
3. L.M. Morato, Phys. Rev. D31, 1982 (1985).
4. The author is grateful to professor E. Nelson for pointing out this fact.