[1]
N. A. Perestyuk and P. V. Feketa, “On Preservation of the Invariant torus for Multifrequency Systems”, Ukr. Mat. Zhurn., vol. 65, no. 11, pp. 1498–1505, Nov. 2013, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/2529