LukovskyI. O. “Variational Method for the Solution of Nonlinear Boundary-Value Problems of the dynamics of Bounded Volumes of Liquid With Variable Boundaries”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 70, no. 1, Jan. 2018, pp. 63-78, https://umj.imath.kiev.ua/index.php/umj/article/view/1542.