VAKAL, Yu. E.; PARASYUK, I. O. Estimation of the number of ultrasubharmonics for a two-dimensional almost autonomous Hamiltonian system periodic in time. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 64, n. 4, p. 463–489, 2012. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/2590. Acesso em: 5 dec. 2025.