Hussien, Ahmed M. “Polynomial Inverse Integrating Factors, First Integral and Non-Existence of Limit Cycles in the Plane for Quadratic Systems”. Science Journal of University of Zakho 5, no. 2 (June 30, 2017): 232–238. Accessed November 26, 2024. https://sjuoz.uoz.edu.krd/index.php/sjuoz/article/view/502.