1. The Four-Color Problem;Ore,1967
2. The Four-Color Problem: Assaults and Conquest;Saaty,1977
3. Every planar map is four colorable. Part I: Discharging;Appel;Illinois Journal of Mathematics,1977
4. Every planar map is four colorable. Part II: Reducibility;Appel;Illinois Journal of Mathematics,1977
5. 75 graph-colouring problems;Toft,1990