AIGNER, M.; CHATYRKO, V. A.; NYAGAHAKWA, V. The Algebra of Semigroups of Sets. MATHEMATICA SCANDINAVICA, [S. l.], v. 116, n. 2, p. 161–170, 2015. DOI: 10.7146/math.scand.a-21158. Disponível em: https://www.mscand.dk/article/view/21158. Acesso em: 18 apr. 2024.