Kolomiyets, V. G., and I. V. Komashinskaya. 1997. “On the Existence of a Solution of a Quasilinear Mixed Problem for a Quasiwave Equation”. Ukrains’kyi Matematychnyi Zhurnal 49 (9), 1182–1185. https://umj.imath.kiev.ua/index.php/umj/article/view/5116.