Dudzyanyi S. I., and PerestyukN. A. “On the Stability of a Trivial Invariant Torus of One Class of Impulsive Systems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 50, no. 3, Mar. 1998, pp. 338–349, https://umj.imath.kiev.ua/index.php/umj/article/view/4935.