[1]
I. V. Protasov, “Ideals of the semigroup of ultrafilters of a topological group”, Ukr. Mat. Zhurn., vol. 47, no. 4, pp. 506–511, Apr. 1995, Accessed: Apr. 13, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5448