PROTASOV, I. V. Ultrafilters and Decompositions of Abelian Groups. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 53, n. 1, p. 85–93, 2001. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4224. Acesso em: 5 dec. 2025.