BerezovskyA. A., MitropolskiyYu. A., and Shkhanukov-LafishevM. Kh. 1996. “Space-Time Localization in Problems With Free Boundaries for a Nonlinear Second-Order Equation”. Ukrains’kyi Matematychnyi Zhurnal 48 (2), 202-11. https://umj.imath.kiev.ua/index.php/umj/article/view/5348.