1. Finding an Independent Set in a Planar Graph;Albertson,1974
2. Graphes et Hypergraphes;Berge,1970
3. An existence theorem for planar maps;Haken;J. Combinatorial Theory B,1973
4. Contribution to the theory of Eulerian polyhedra;Kotzig;Mat. Fyz. Casopis Sav.,1955
5. The Four Color Problem;Ore,1967