Tunç, C. ., and S. . Erdur. “On the Existence of Periodic Solutions to Certain Non-Linear Differential Equations of Third Order: On the Existence of Periodic Solutions”. Proceedings of the Pakistan Academy of Sciences: A. Physical and Computational Sciences, vol. 54, no. 2, Apr. 2021, pp. 207–218, http://ppaspk.org/index.php/PPAS-A/article/view/251.