1. Every planar map is four colorable. Part I. Discharging;Appel;Illinois J. Math.,1977
2. Every planar map is four colorable. Part II. Reducibility;Appel;Illinois J. Math.,1977
3. Tait's theorem for graphs with crossing number at most one;Jaeger;Ars Combin.,1980
4. The four colour theorem;Robertson;J. Combin. Theory Ser. B,1997