Богачев, В. Е., И. К. Кириченко, Н. Н. Чеканова, and Н. А. Чеканов. “Investigation of Nonlinear Hamiltonian Systems by the Normal Form of the Birkhoff-Gustavson”. Bulletin of V.N. Karazin Kharkiv National University, Series «Mathematical Modeling. Information Technology. Automated Control Systems», Vol. 26, no. 1156, May 2015, pp. 17-28, https://periodicals.karazin.ua/mia/article/view/14210.