FELLER, M. N. Riquier problem for a nonlinear equation resolved with respect to the iterated Levi Laplacian. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 50, n. 11, p. 1574–1577, 1998. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4813. Acesso em: 5 dec. 2025.