Kulik, V. L. “Construction of Lyapunov Functions in the Theory of Regular Linear Extensions of Dynamical Systems on a Torus”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 67, no. 10, Oct. 2015, pp. 1358-65, https://umj.imath.kiev.ua/index.php/umj/article/view/2072.