Larin, D. V., and I. V. Skrypnik. 1998. “Principle of Additivity in Averaging of Degenerate Nonlinear Dirichlet Problems”. Ukrains’kyi Matematychnyi Zhurnal 50 (1): 118–135. https://umj.imath.kiev.ua/index.php/umj/article/view/4977.