AIGNER, Mats; CHATYRKO, Vitalij A.; NYAGAHAKWA, Venuste. The Algebra of Semigroups of Sets. Mathematica Scandinavica, Berkeley, CA, United States, v. 116, n. 2, p. 161–170, 2015. DOI: 10.7146/math.scand.a-21158. Disponível em: https://journals.msp.org/mscand/article/view/394. Acesso em: 24 jul. 2026.