CHRISTENSEN, J. P. R. Borel Structures in Groups and Semigroups. MATHEMATICA SCANDINAVICA, [S. l.], v. 28, p. 124–128, 1971. DOI: 10.7146/math.scand.a-11010. Disponível em: https://www.mscand.dk/article/view/11010. Acesso em: 2 may. 2024.