Luo, Liping, et al. “Infinitely Many Fast Homoclinic Solutions for Some Second-Order Nonautonomous Systems”. Ukrains’kyi Matematychnyi Zhurnal, vol. 66, no. 3, Mar. 2014, pp. 404–414, https://umj.imath.kiev.ua/index.php/umj/article/view/2142.