Imagine that you’re a stingy cartographer and that you want to make a colored map of the united states. Because you’re stingy, you want to avoid spending money on ink. You have to color the map so that no two adjacent states are the same color—otherwise you wouldn’t be able to tell them apart! If you want to buy the fewest colored pens possible, how many colors must you use to make your map? Very early on, mathematicians guessed that the answer was four colors. However, no one could prove it. An example map is in the tittle figure,