Some existence theorems are obtained for periodic solutions of the subquadratic second order systems by the minimax methods in critical point theory. © 2003 Elsevier Inc. All rights reserved.
Tang, C. L., & Wu, X. P. (2003). Notes on periodic solutions of subquadratic second order systems. Journal of Mathematical Analysis and Applications, 285(1), 8–16. https://doi.org/10.1016/S0022-247X(02)00417-1