Lungu, Nicolaie. “Conditions of Existence in the Future for Liénard-Rayleight-Type Systems of Equations”. Anal. Numér. Théor. Approx. 17, no. 1 (February 1, 1988): 25–27. Accessed April 11, 2025. https://ictp.acad.ro/jnaat/journal/article/view/1988-vol17-no1-art4.