TARSKI, Alfred. Remarks on direct products of commutative semigroups. MATHEMATICA SCANDINAVICA, [S. l.], v. 5, p. 218–223, 1957. DOI: 10.7146/math.scand.a-10497. Disponível em: https://tidsskrift.dk/math/article/view/10497. Acesso em: 14 aug. 2026.