Dudzyanyi, S. I., and N. A. Perestyuk. “On the Stability of an Invariant Torus”. Ukrains’kyi Matematychnyi Zhurnal 49, no. 9 (September 25, 1997): 1212–1222. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5120.