ZHUCHOK, A. V.; KOPPITZ, J. Free products of $n$-tuple semigroups. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 70, n. 11, p. 1484–1498, 2018. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/1652. Acesso em: 5 dec. 2025.