Vakal, Y. 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, Vol. 64, no. 4, Apr. 2012, pp. 463-89, https://umj.imath.kiev.ua/index.php/umj/article/view/2590.