Zelinskii, Yu. B., and M. V. Stefanchuk. “Generalizations of the Shadow Problem”. Ukrains’kyi Matematychnyi Zhurnal 68, no. 6 (June 25, 2016): 757-763. Accessed April 25, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/1876.