An estimate of the period of periodical solution of an autonomous system of differential equations. Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, [S. l.], v. 91, p. 107–114, 1999. Disponível em: https://annual.uni-sofia.bg/index.php/fmi/article/view/273. Acesso em: 11 jan. 2026.