PyatetskayaE. V. “Invariant Tori of Linear Extensions Which Do Not Have Green’s Functions”. Ukrains’kyi Matematychnyi Zhurnal 45, no. 4 (April 25, 1993): 520–524. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5839.