KROM, Melven; KROM, Myren. Free Subgroups and Folners Conditions. Mathematica Scandinavica, Berkeley, CA, United States, v. 32, p. 242–244, 1973. DOI: 10.7146/math.scand.a-11458. Disponível em: https://journals.msp.org/mscand/article/view/2150. Acesso em: 25 jul. 2026.