Any planar map can be colored with four colors so that no two adjacent regions have the same color. The picture to the right in Figure 1 also represents a planar map, and it has been colored with the four colors red , blue , purple , and green . Note also that the regions A and B only share a point as a common boundary and so are not considered adjacent. Cases like this make this theorem difficult to solve.
Coloring is a huge problem in mathematics.
The history of the Four Color Theorem can be found here .