KALINOWSKI, J. On the Sign of Solutions of Systems of Ordinary Differential Equations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 54, n. 9, p. 1282–1283, 2002. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4166. Acesso em: 5 dec. 2025.