PASCHKE, William L. A numerical invariant for finitely generated groups via actions on graphs. Mathematica Scandinavica, Berkeley, CA, United States, v. 72, p. 148–160, 1993. DOI: 10.7146/math.scand.a-12442. Disponível em: https://journals.msp.org/mscand/article/view/1185. Acesso em: 25 jul. 2026.