Feller, M. N. “Boundary-Value Problems for a Nonlinear Hyperbolic Equation With Divergent Part and Levy Laplacian”. Ukrains’kyi Matematychnyi Zhurnal, vol. 64, no. 2, Feb. 2012, pp. 237-44, https://umj.imath.kiev.ua/index.php/umj/article/view/2570.