KATANI, R.; MICCOLI, M. M. Upper Subrings of a Ring. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 53, n. 10, p. 1427–1428, 2001. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4361. Acesso em: 6 dec. 2025.