LI, Jin; LUO, Jianlin; WANG, Zaihong. RETRACTED ARTICLE: Periodic Solutions of Second Order Impulsive Differential Equations at Resonance via Variational Approach. Mathematical Modelling and Analysis, [S. l.], v. 19, n. 5, p. 664–675, 2014. DOI: 10.3846/13926292.2014.980864. Disponível em: https://journals.vilniustech.lt/index.php/MMA/article/view/3330. Acesso em: 2 jun. 2025.