FIHEL, I.R.; GUTIK, O.V. On the closure of the extended bicyclic semigroup. Carpathian Mathematical Publications, [S. l.], v. 3, n. 2, p. 131–157, 2011. Disponível em: https://ktipe.pnu.edu.ua/index.php/cmp/article/view/3075. Acesso em: 5 jun. 2025.