FellerM. N. “Riquier Problem for a Nonlinear Equation Resolved With Respect to the Iterated Levi Laplacian”. Ukrains’kyi Matematychnyi Zhurnal 50, no. 11 (November 25, 1998): 1574–1577. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4813.