Li, Jin, Jianlin Luo, and Zaihong Wang. 2015. “Retraction Notice to ‘Periodic Solutions of Second Order Impulsive Differential Equations at Resonance via Variational Approach’ [Math. Model. Anal. 19(5):664–675, 2014]”. Mathematical Modelling and Analysis 20 (2):289. https://doi.org/10.3846/13926292.2015.1024973.