2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/220827It is shown that the generating function for tree graphs in the "in-in" formalism may be calculated by solving the classical equations of motion subject to certain constraints. This theorem is illustrated by application to the evolution of a single inflaton field in a Robertson--Walker background.Version accepted for publication in Phys. Rev. D. 22 pages. Some changes to references and clarification in IntroductionHigh Energy Physics - TheoryAstrophysicsGeneral Relativity and Quantum CosmologyHigh Energy Physics - Phenomenologytext