Lungu, Nicolae. “On the Number of Limit Cycles for Systems With Several Singular Points”. Anal. Numér. Théor. Approx. 16, no. 1 (February 1, 1987): 47–50. Accessed November 22, 2024. https://ictp.acad.ro/jnaat/journal/article/view/1987-vol16-no1-art7.