Koval’chuk, P. S., and V. D. Kubenko. “Application of the Averaging Method to the Investigation of Nonlinear Wave Processes in Elastic Systems With Circular Symmetry”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 53, no. 10, Oct. 2001, pp. 1358-67, https://umj.imath.kiev.ua/index.php/umj/article/view/4355.