Pereverzev, S. V., and S. G. Solodkii. “On One Approach to the Discretization of the lavrent’ev Method”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 2, Feb. 1996, pp. 212-9, https://umj.imath.kiev.ua/index.php/umj/article/view/5349.