“…Recently there has been interest in studying field theory using two-particle irreducible (2-PI) methods [1] in both finite temperature [2,3,4] and non-equilibrium situations [5,6,7,8,9,10,11,12,13,14]. The value of the 2-PI formalism for non-equilibrium problems is that it allows one to make approximations that go beyond the Hartree or large-N approximation without encountering the serious problems of secularity found in a straightforward expansion about the Hartree or leadingorder large-N approximation using the generating functional or, equivalently, the one-particle irreducible (1-PI) action [15].…”