1. Buică, A., Françoise, J.P., Llibre, J.: Periodic solutions of nonlinear periodic differential systems with a small parameter. Commun. Pure Appl. Anal. 6, 103–111 (2007)
2. de Bustos, M.T., Guirao, J.L.G., Vera, J.A., Vigo-Aguiar, J.: Periodic orbits and $\mathcal{C}^{1}$ -integrability in the planar Stark–Zeeman problem. J. Math. Phys. 53, 082701 (2012)
3. Guirao, J.L.G., Llibre, J., Vera, J.A.: Generalized van der Waals Hamiltonian: periodic orbits and C 1 nonintegrability. Phys. Rev. E, Stat. Nonlinear Soft Matter Phys. 85, 036603 (2012)
4. Guirao, J.L.G., Llibre, J., Vera, J.A.: On the dynamics of the rigid body with a fixed point: periodic orbits and integrability. Nonlinear Dyn. (2013). doi: 10.1007/s11071-013-0797-8
5. Malkin, I.G.: Some Problems of the Theory of Nonlinear Oscillations. Gosudarstv. Izdat. Tehn.-Teor. Lit, Moscow (1956). In Russian