ROBINSON, D. W. Commutators and generators II. MATHEMATICA SCANDINAVICA, [S. l.], v. 64, p. 87–108, 1989. DOI: 10.7146/math.scand.a-12249. Disponível em: https://www.mscand.dk/article/view/12249. Acesso em: 3 may. 2024.