KalitaE. A. “On Nonlinear Higher Order Elliptic Systems Positive in the First Derivatives”. Ukrains’kyi Matematychnyi Zhurnal 45, no. 7 (July 25, 1993): 942–947. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5887.