The number of limit cycles of a quintic Hamiltonian system with perturbation. Atabaigi, Ali, Nyamoradi, Nemat, Zangeneh, Hamid R.Z. (2008) Balkan Journal of Geometry and its Applications (BJGA)
The period function near a polycycle with two semi-hyperbolic vertices Angélica Mansilla, Mariana Saavedra (2001) Annales mathématiques Blaise Pascal