1. Every planar map is four-colourable;Appel;Illinois J. Math.,1977
2. Graphs and Hypergraphs;Berge,1973
3. A generalization of Gilmore's theorem;Berge,1975
4. Geometry of Geodesics;Busemann,1955
5. Some elementary properties of interval convexities;Calder;J. London Math. Soc. (2),1971