BAUMSLAG, G.; BOONE, W. W.; NEUMANN, B. H. Some Unsolvable Problems about Elements and Subgroups of Groups. Mathematica Scandinavica, Berkeley, CA, United States, v. 7, p. 191–201, 1959. DOI: 10.7146/math.scand.a-10572. Disponível em: https://journals.msp.org/mscand/article/view/2770. Acesso em: 8 sep. 2026.