PASCHKE, William L. A numerical invariant for finitely generated groups via actions on graphs. MATHEMATICA SCANDINAVICA, [S. l.], v. 72, p. 148–160, 1993. DOI: 10.7146/math.scand.a-12442. Disponível em: https://tidsskrift.dk/math/article/view/12442. Acesso em: 13 aug. 2026.