 We can expand the exponential as a Taylor series to obtain the perturbation series formula_48.This is the same procedure as in equilibrium diagrammatic perturbation theory, but with the important difference that both forward and reverse contour branches are included.If, as is often the case, formula_49 is a polynomial or series as a function of the elementary fields formula_50, we can organize this perturbation series into monomial terms and apply all possible Wick pairings to the fields in each monomial, obtaining a summation of Feynman diagrams.