Vakal, Yu. E., and I. O. Parasyuk. “Estimation of the Number of Ultrasubharmonics for a Two-Dimensional Almost Autonomous Hamiltonian System Periodic in Time”. Ukrains’kyi Matematychnyi Zhurnal 64, no. 4 (April 25, 2012): 463-489. Accessed April 26, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2590.