CHRISTENSEN, Jens Peter Reus. 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://tidsskrift.dk/math/article/view/11010. Acesso em: 14 aug. 2026.