PetrenkoO. V., and ProtasovI. V. “Balleans and <i>G</I&gt; -Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 64, no. 3, Mar. 2012, pp. 344-50, https://umj.imath.kiev.ua/index.php/umj/article/view/2580.