IGNATYEV, Alexander. “On the Existence of a Periodic Solution of the Liénard System”. Studia Universitatis Babeș-Bolyai Mathematica 66, no. 1 (March 30, 2021): 47–53. Accessed February 5, 2025. https://studia.reviste.ubbcluj.ro/index.php/subbmathematica/article/view/1239.