Bazhura, B. P., & Samoilenko, A. M. (1994). Investigation of a dynamical system in a neighborhood of an invariant toroidal manifold in the general case. Ukrains’kyi Matematychnyi Zhurnal, 46(10), 1399–1408. https://umj.imath.kiev.ua/index.php/umj/article/view/5620