Iskakov, R. S., and V. N. Pavlenko. “Continuous Approximations of Discontinuous Nonlinearities of Semilinear Elliptic-Type Equations”.
Ukrains’kyi Matematychnyi Zhurnal 51, no. 2 (February 25, 1999): 224–233. Accessed April 14, 2025.
https://umj.imath.kiev.ua/index.php/umj/article/view/4601.