BittievK. R. “Space-Time Localization in One-Dimensional Problems With Free Boundary for Nonlinear Second-Order Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 50, no. 11, Nov. 1998, pp. 1554–1558, https://umj.imath.kiev.ua/index.php/umj/article/view/4808.