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2. G. t’ Hooft, Phys.Rev.Lett. 37, 8 (1976)
3. G. t’ Hooft Phys.Rev. D14, 3432 (1976).
4. For a clear exposition of the functional integral approach see S. Colemann, “The Uses of Instantons”, Lectures at the 1977 International School “Ettore Majorana”.
5. E. Nelson, Phys.Rev. 150, 1079 (1966)