JESSEN, B.; KARPF, J.; THORUP, A. Some Functional Equations in Groups and Rings. Mathematica Scandinavica, Berkeley, CA, United States, v. 22, p. 257–265, 1968. DOI: 10.7146/math.scand.a-10889. Disponível em: https://journals.msp.org/mscand/article/view/2416. Acesso em: 25 jul. 2026.