2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/187223A derivation is given of the Feynman rules to be used in the perturbative computation of the Green's functions of a generic quantum many-body theory when the action which is being perturbed is not necessarily quadratic. Some applications are discussed.Extended revised version. RevTex, 19 pages, 10 figuresHigh Energy Physics - Theorytext