Bazalii, B. V., and M. V. Krasnoshchok. “On One Problem With Free Boundary for a Nonlinear System”. Ukrains’kyi Matematychnyi Zhurnal, vol. 48, no. 9, Sept. 1996, pp. 1155–1165, https://umj.imath.kiev.ua/index.php/umj/article/view/5231.