Kostin, A. V., and S. A. Shchegolev. “On Solutions of a Second-Order Quasilinear Differential System Representable by Fourier Series With Slowly Varying Parameters”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 50, no. 5, May 1998, pp. 654–664, https://umj.imath.kiev.ua/index.php/umj/article/view/4903.