2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/41402Based on the closed time path formalism, a new Feynman rule for directly calculating the retarded and advanced Green functions is deduced. This Feynman rule is used to calculate the two-point self-energy and three-point vertex correction in $ϕ^3$ theory. The generalized fluctuation-dissipation theorem for three-point nonlinear response function is verified.Text and formula are revisedHigh Energy Physics - Phenomenologytext