WANG, Zaihong; MA, Tiantian. A continuation lemma and the existence of periodic solutions of perturbed planar Hamiltonian systems with sub-quadratic potentials. Topological Methods in Nonlinear Analysis, [S. l.], v. 52, n. 2, p. 693–706, 2018. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2018.037. Acesso em: 8 jul. 2025.