2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/73076The phase--portrait of the second order differential equation: $$\ddot x+\sum_{l=0}^nf_l(x) \dot x^l=0 ,$$ is studied. Some results concerning existence, non--existence and uniqueness of limit cycles are presented. Among these, a generalization of the classical Massera uniqueness result is proved.5 figuresClassical Analysis and ODEsDynamical Systems30C07text