BISGAARD, Torben Maack; SAKAKIBARA, Nobuhisa. A reduction of the problem of characterizing perfect semigroups. Mathematica Scandinavica, Berkeley, CA, United States, v. 91, n. 1, p. 55–66, 2002. DOI: 10.7146/math.scand.a-14378. Disponível em: https://journals.msp.org/mscand/article/view/841. Acesso em: 24 jul. 2026.