PROTASOV, I. V. Ideals of the semigroup of ultrafilters of a topological group. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 47, n. 4, p. 506–511, 1995. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5448. Acesso em: 14 apr. 2025.