Mitropolskiy, Y. A., Khoma, N. H., & Khoma, G. P. (1998). Conditions of solvability of quasilinear periodic boundary-value problems for hyperbolic equations of the second order. Ukrains’kyi Matematychnyi Zhurnal, 50(6), 818–821. https://umj.imath.kiev.ua/index.php/umj/article/view/4892