This article provides sufficient conditions for the existence of periodic solutions with nonconstant sign in a family of polynomial, non-auto-nomous, firrst-order diferential equations that arise as a generalization of the Abel equation of the second kind.
Olm, J.; Ros, X. Existence of periodic solutions with nonconstant sign in a class of generalized abel equations. "Discrete and continuous dynamical systems. Series A", Abril 2013, vol. 33, núm. 4, p. 1603-1614.