Li, Jin, Jianlin Luo, and Zaihong Wang. “RETRACTED ARTICLE: Periodic Solutions of Second Order Impulsive Differential Equations at Resonance via Variational Approach”. Mathematical Modelling and Analysis 19, no. 5 (November 1, 2014): 664-675. Accessed December 2, 2024. https://mma.vgtu.lt/index.php/MMA/article/view/3330.