BISGAARD, Torben Maack. Semiperfect finitely generated abelian semigroups without involution. Mathematica Scandinavica, Berkeley, CA, United States, v. 91, n. 2, p. 285–319, 2002. DOI: 10.7146/math.scand.a-14391. Disponível em: https://journals.msp.org/mscand/article/view/828. Acesso em: 24 jul. 2026.