GRIGORIAN, S. A.; KHORKOVA, T. A. On $w$-extensions of the abelian semigroups. Armenian Journal of Mathematics. Armenian Journal of Mathematics, Yerevan, Armenia, v. 2, n. 2, p. 73–76, 2009. Disponível em: https://armjmath.sci.am/index.php/ajm/article/view/54. Acesso em: 3 apr. 2025.