BerezovskyA. A., MitropolskiyY. A., and Shkhanukov-LafishevM. K. “Space-Time Localization in Problems With Free Boundaries for a Nonlinear Second-Order Equation”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 2, Feb. 1996, pp. 202-11, https://umj.imath.kiev.ua/index.php/umj/article/view/5348.