KOESTER, G. 4-critical 4-valent planar graphs constructed with crowns. MATHEMATICA SCANDINAVICA, [S. l.], v. 67, p. 15–22, 1990. DOI: 10.7146/math.scand.a-12316. Disponível em: https://tidsskrift.dk/math/article/view/12316. Acesso em: 14 aug. 2026.