2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/110239Given a planar differential system with a first integral, we show how to find a normalizer. For systems with a center, we give an integral formula for the derivative of its period function.Dynamical SystemsClassical Analysis and ODEs34C25; 34C14Normalizers of planar systems with known first integralstext