Piskunov , A. G. “Power of Open $\sigma$-Compact Sets in the Space of Non-Compact Subgroups of the Topological Group”. Ukrains’kyi Matematychnyi Zhurnal, vol. 40, no. 6, Nov. 1988, pp. 803-7, https://umj.imath.kiev.ua/index.php/umj/article/view/9297.