TARSKI, Alfred. Remarks on direct products of commutative semigroups. Mathematica Scandinavica, Berkeley, CA, United States, v. 5, p. 218–223, 1957. DOI: 10.7146/math.scand.a-10497. Disponível em: https://journals.msp.org/mscand/article/view/2824. Acesso em: 25 jul. 2026.