SAMOILENKO , A. M.; KULIK , V. L. Concerning existence of the Lyapunov sign-variable functions for linear systems of differential equations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 44, n. 1, p. 100–106, 1992. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/7800. Acesso em: 14 apr. 2025.