BISGAARD, T. M. Semiperfect finitely generated abelian semigroups without involution. MATHEMATICA SCANDINAVICA, [S. l.], v. 91, n. 2, p. 285–319, 2002. DOI: 10.7146/math.scand.a-14391. Disponível em: https://www.mscand.dk/article/view/14391. Acesso em: 2 may. 2024.