[1] M. R. Grossinho, R. Martins, Subharmonic oscillations for some second-order differential equations without Landsman-Lazer conditions, Proceedings of the International Conference Dynamical Systems and Differential Equations, May 2000, Atlanta, published as an added volume of Discrete and Continuous Dynamical Systems.
[2] R. Martins, Existence of periodic solutions for second-order differential equations with singularities and the strong force condition, J. Math. Anal. Appl., Volume 317, Issue 1, 1 May 2006, 1-13.
