NIELSEN, Jacob. 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://tidsskrift.dk/math/article/view/10423. Acesso em: 14 aug. 2026.