Piskunov , A. G. “Power of Open $\sigma$-Compact Sets in the Space of Non-Compact Subgroups of the Topological Group”. Ukrains’kyi Matematychnyi Zhurnal 40, no. 6 (November 1, 1988): 803–807. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/9297.