Zheng, Nannan, and Zaihong Wang. “Infinitely Many Periodic Solutions of Duffing Equations under Integral Condition”. Topological Methods in Nonlinear Analysis 57, no. 1 (December 18, 2020): 297–315. Accessed July 3, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2020.017.