BISGAARD, T. M.; SAKAKIBARA, N. A reduction of the problem of characterizing perfect semigroups. MATHEMATICA SCANDINAVICA, [S. l.], v. 91, n. 1, p. 55–66, 2002. DOI: 10.7146/math.scand.a-14378. Disponível em: https://www.mscand.dk/article/view/14378. Acesso em: 2 may. 2024.