NIELSEN, J. A basis for subgroups of free groups. MATHEMATICA SCANDINAVICA, [S. l.], v. 3, p. 31–43, 1955. DOI: 10.7146/math.scand.a-10423. Disponível em: https://www.mscand.dk/article/view/10423. Acesso em: 19 apr. 2024.