Samoilenko, A. M. “On Some Problems in Perturbation Theory of Smooth Invariant Tori of Dynamical Systems”. Ukrains’kyi Matematychnyi Zhurnal, vol. 46, no. 12, Dec. 1994, pp. 1665–1699, https://umj.imath.kiev.ua/index.php/umj/article/view/5577.