FELLER, M. N. The riquier problem for a nonlinear equation unresolved with respect to the Lévy iterated laplacian. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 51, n. 3, p. 423–427, 1999. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4627. Acesso em: 5 dec. 2025.