We give sufficient conditions in terms of the Melnikov functions in order that an analytic or a polynomial differential system in the real plane has a period annulus. We study the first nonzero Melnikov function of the analytic differential systems in the real plane obtained by perturbing a Hamiltonian system having either a nondegenerate center, a heteroclinic cycle, a homoclinic cycle, or three cycles obtained connecting the four separatrices of a saddle. All the singular points of these cycles are hyperbolic saddles. Finally, using the first nonzero Melnikov function we give a new proof of a result of Roussarie on the finite cyclicity of the homoclinic orbit of the integrable system when we perturb it inside the class of analytic differential systems.
Li, W., Llibre, J., & Zhang, X. (2004). Melnikov functions for period annulus, nondegenerate centers, heteroclinic and homoclinic cycles. Pacific Journal of Mathematics, 213(1), 49-77. https://doi.org/10.2140/pjm.2004.213.49