BattelliF., and PalmerK. J. “Connections to Fixed Points and Sil’nikov Saddle-Focus Homoclinic Orbits in Singularly Perturbed Systems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 60, no. 1, Jan. 2008, pp. 28–55, https://umj.imath.kiev.ua/index.php/umj/article/view/3135.