PROTASOV, I. V.; SLOBODIANIUK, S. V. Ultrafilters on balleans. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 67, n. 12, p. 1698–1706, 2015. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/2102. Acesso em: 6 dec. 2025.