Gor’kavyiV. A., DiskantV. I., and MilkaA. D. “International Conference on Geometry "In the Large"”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 52, no. 7, July 2000, pp. 1007-8, https://umj.imath.kiev.ua/index.php/umj/article/view/4503.