BAUMSLAG, G.; BOONE, W. W.; NEUMANN, B. H. Some Unsolvable Problems about Elements and Subgroups of Groups. MATHEMATICA SCANDINAVICA, [S. l.], v. 7, p. 191–201, 1959. DOI: 10.7146/math.scand.a-10572. Disponível em: https://www.mscand.dk/article/view/10572. Acesso em: 4 may. 2024.