1. Cheng, Z., Ren, J.: Existence of periodic solution for fourth-order Liénard type p-Laplacian generalized neutral differential equation with variable parameter. J. Appl. Anal. Comput. 5, 704–720 (2015)
2. Cheng, Z., Ren, J.: Periodic solution for second order damped differential equations with attractive-repulsive singularities. Rocky Mt. J. Math. 48, 753–768 (2018)
3. Cheung, W., Ren, J.: Periodic solutions for p-Laplacian Liénard equation with a deviating argument. Nonlinear Anal. 59, 107–120 (2004)
4. Cheung, W., Ren, J.: On the existence of periodic solutions for p-Laplacian generalized Liénard equation. Nonlinear Anal. 60, 65–75 (2005)
5. Hakl, R., Zamora, M.: Periodic solution to second-order indefinite singular equations. J. Differ. Equ. 263, 451–469 (2017). https://doi.org/10.1016/j.jde.2017.02.044