Kolesov, A. Y., and N. K. Rozov. “New Methods for the Investigation of Periodic Solutions in Ring Systems of Unidirectionally Coupled Oscillators”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 65, no. 1, Jan. 2013, pp. 82-102, https://umj.imath.kiev.ua/index.php/umj/article/view/2406.