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://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/14045. Acesso em: 5 jul. 2025.