Fonda, Alessandro, and Luca Ghirardelli. “Multiple Periodic Solutions of Hamiltonian Systems in the Plane”. Topological Methods in Nonlinear Analysis 36, no. 1 (April 23, 2010): 27–38. Accessed April 25, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2010.026.