Maslyuchenko, V. K., & Fotii, O. H. (2007). Constancy of upper-continuous two-valued mappings into the Sorgenfrey line. Ukrains’kyi Matematychnyi Zhurnal, 59(8), 1034–1039. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/3367