CHRISTENSEN, Erik. Extensions of derivations II. Mathematica Scandinavica, Berkeley, CA, United States, v. 50, p. 111–122, 1982. DOI: 10.7146/math.scand.a-11948. Disponível em: https://journals.msp.org/mscand/article/view/1701. Acesso em: 25 jul. 2026.