1. On the geographical problem of the four colours;Alfred B Kempe;American journal of mathematics,1879
2. Map color theorem;P J Heawood;Quart. J. Pure Appl. Math,1890
3. The reducibility of maps;D George;American Journal of Mathematics,1913
4. Every planar map is four colorable part i. discharging;Appel Kenneth;Illinois Journal of Mathematics,1977
5. Every planar map is four colorable. part ii: Reducibility;Kenneth Appel;Illinois Journal of Mathematics,1977