BoichukО. A. “A Criterion for the Existence of the Unique Invariant Torus of a Linear Extension of Dynamical Systems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, no. 1, Jan. 2007, pp. 3–13, https://umj.imath.kiev.ua/index.php/umj/article/view/3288.