Bazhura, B. P., and A. M. Samoilenko. “Investigation of a Dynamical System in a Neighborhood of an Invariant Toroidal Manifold in the General Case”. Ukrains’kyi Matematychnyi Zhurnal, vol. 46, no. 10, Oct. 1994, pp. 1399–1408, https://umj.imath.kiev.ua/index.php/umj/article/view/5620.