Dudzyanyi, .S.I. and Perestyuk, N.A. (1997) “On the stability of an invariant torus”, Ukrains’kyi Matematychnyi Zhurnal, 49(9), pp. 1212–1222. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/5120 (Accessed: 6 December 2025).