Keywords: Prism-vf-color.svg coloring the vertices and faces of a planar graph may require six colors ” for instance in this triangular prism no three of the 11 features may take the same color Ringel conjectured in 1965 that six colors always suffice; this was proven in 1984 by Borodin This coloring problem formed the inspiration for 1-planar graphs 2013-01-20 22 38 51 own David Eppstein cc-zero Files by User David Eppstein from en wikipedia Planar graphs Prisms geometry Graph coloring; face Graph coloring; vertex |