Feller, M. N. “On a Nonlinear Equation Unsolved With Respect to the Levy Laplacian”.
Ukrains’kyi Matematychnyi Zhurnal
48, no. 5 (May 25, 1996): 719–721. Accessed April 27, 2025.
https://umj.imath.kiev.ua/index.php/umj/article/view/5307
.