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