FRÖBERG, Ralf. A study of graded extremal rings and of monominal rings. Mathematica Scandinavica, Berkeley, CA, United States, v. 51, p. 22–34, 1982. DOI: 10.7146/math.scand.a-11961. Disponível em: https://journals.msp.org/mscand/article/view/1687. Acesso em: 25 jul. 2026.