Feller, M. N., Ya. I. Belopolskaya, and S. Albeverio. 2005. “Riquier Problem for Nonlinear Elliptic Equations With Lévy Laplacians”. Methods of Functional Analysis and Topology 11 (1): 1-9. https://zen.imath.kiev.ua/index.php/mfat/article/view/261.