Kapustyan, V. E. “The Asymptotics of Constrained Control in Optimal Singular Parabolic Problems”. Ukrains’kyi Matematychnyi Zhurnal, vol. 45, no. 10, Oct. 1993, pp. 1345–1355, https://umj.imath.kiev.ua/index.php/umj/article/view/5939.