LarinD. V., and SkrypnikI. V. “Principle of Additivity in Averaging of Degenerate Nonlinear Dirichlet Problems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 50, no. 1, Jan. 1998, pp. 118–135, https://umj.imath.kiev.ua/index.php/umj/article/view/4977.