PROTASOV, I. V. Ideals and free Pairs in the semigroup β ℤ. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 49, n. 4, p. 573–580, 1997. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5033. Acesso em: 5 dec. 2025.