Yuji Liu, and Liuman Ou. “The Existence of Periodic Solutions for Second Order Functional Differential Equations Caused by Impulses Effects”. Jordan Journal of Mathematics and Statistics 4, no. 3 (May 18, 2025): 219–248. Accessed September 10, 2025. https://jjms.yu.edu.jo/index.php/jjms/article/view/1150.