Boichuk О. A. “A Condition for the Existence of a Unique Green–Samoilenko Function for the Problem of Invariant Torus”. Ukrains’kyi Matematychnyi Zhurnal 53, no. 4 (April 25, 2001): 556–559. Accessed April 7, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4277.