Y. E. Vakal and I. O. Parasyuk, “Estimation of the number of ultrasubharmonics for a two-dimensional almost autonomous Hamiltonian system periodic in time”, Ukr. Mat. Zhurn., vol. 64, no. 4, pp. 463–489, Apr. 2012, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/2590