COHN, J. H. E. On $n$-Sum Groups. Mathematica Scandinavica, Berkeley, CA, United States, v. 75, p. 44–58, 1994. DOI: 10.7146/math.scand.a-12501. Disponível em: https://journals.msp.org/mscand/article/view/1172. Acesso em: 24 jul. 2026.