Gritsans, Armands, and Inara Yermachenko. “On the Maximum Number of Period Annuli for Second Order Conservative Equations”. Mathematical Modelling and Analysis 26, no. 4 (November 26, 2021): 612-630. Accessed April 25, 2024. https://journals.vilniustech.lt/index.php/MMA/article/view/13979.