Lukovsky, I. O., & Timokha, A. N. (1993). Variational formulations of nonlinear boundary-value problems with a free boundary in the theory of interaction of surface waves with acoustic fields. Ukrains’kyi Matematychnyi Zhurnal, 45(12), 1642–1652. https://umj.imath.kiev.ua/index.php/umj/article/view/5972