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 “RETRACTED ARTICLE: Periodic Solutions of Second Order Impulsive Differential Equations at Resonance via Variational Approach”.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 November 25, 2024. https://jau.vgtu.lt/index.php/MMA/article/view/3330.