Limarchenko, O. S. “Specific Features of Application of Perturbation Techniques in Problems of Nonlinear Oscillations of a Liquid With Free Surface in Cavities of Noncylindrical Shape”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, no. 1, Jan. 2007, pp. 44–70, https://umj.imath.kiev.ua/index.php/umj/article/view/3291.