TVERBERG, Helge. On a four-colour theorem. Mathematica Scandinavica, Berkeley, CA, United States, v. 39, p. 267–270, 1976. DOI: 10.7146/math.scand.a-11661. Disponível em: https://journals.msp.org/mscand/article/view/1986. Acesso em: 25 jul. 2026.