Maslyuchenko, V. K., and O. I. Filipchuk. “Discontinuity Points of Separately Continuous Mappings With at Most Countable Set of Values”. Ukrains’kyi Matematychnyi Zhurnal, vol. 71, no. 6, June 2019, pp. 801-7, https://umj.imath.kiev.ua/index.php/umj/article/view/1477.