BARREIRA, Luis; LLIBRE, Jaume; VALLS, Claudia. Periodic orbits near equilibria via averaging theory of second order. Mathematical Modelling and Analysis, [S. l.], v. 17, n. 5, p. 715–731, 2012. DOI: 10.3846/13926292.2012.736090. Disponível em: https://journals.vilniustech.lt/index.php/MMA/article/view/4905. Acesso em: 17 jul. 2025.