IGNATYEV, Alexander. On the existence of a periodic solution of the Liénard system. Studia Universitatis Babeș-Bolyai Mathematica, [S. l.], v. 66, n. 1, p. 47–53, 2021. DOI: 10.24193/subbmath.2021.1.04. Disponível em: https://studia.reviste.ubbcluj.ro/index.php/subbmathematica/article/view/1239. Acesso em: 5 feb. 2025.