Parasyuk, I. O., B. V. Repeta, and A. M. Samoilenko. “Dynamical Bifurcation of Multifrequency Oscillations in a Fast-Slow System”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 67, no. 7, July 2015, pp. 890-15, http://umj.imath.kiev.ua/index.php/umj/article/view/2032.