Valiуeva D.G. On the autonomous systems on a plane and limit cycles. Bulletin of the Karaganda University. Mathematics Series, [S. l.], v. 79, n. 3, p. 12–16, 2015. Disponível em: http://mts.buketov.edu.kz/index.php/mathematics-vestnik/article/view/41. Acesso em: 7 sep. 2025.