LIGĘZA, Jan. Existence of positive periodic solutions of some differential equations of order n (n≥2). Annales Mathematicae Silesianae, [S. l.], v. 23, p. 61–81, 2009. Disponível em: https://journals.us.edu.pl/index.php/AMSIL/article/view/14045. Acesso em: 5 dec. 2025.