“On Limit Cycles of Planar Dynamical System Via Dulac- Cherkas Function”. Journal of Zankoy Sulaimani - Part A 17, no. 2 (January 25, 2015): 45–50. Accessed February 22, 2024. https://jzs.univsul.edu.iq/index.php/jzs/article/view/jzs-10379.