TIL: ELI5

The Four Color Theorem


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Imagine that you are given a map, and your job is to color each region in such a way that no two adjoining regions have the same color. How many different colors would you need to successfully complete this task without any two adjacent regions being the same color? The Four Color Theorem answers that question.

Introduced in the mid-19th century, the Four Color Theorem states that any geographical map in a plane can be colored using only four colors. The catch here is that no two regions sharing a common boundary can have the same color. You might be thinking, "That can't be. Surely, there must be some map that requires more than four colors." But, no matter how complex a geographical map gets, just four distinct colors are enough to paint it without having similar colors touch.
Let's put it this way, we represent each region of the map as a lump of clay connected by strings to its neighboring lumps. Each string represents a shared boundary. If we can successfully color that bundle of clay and strings with just four colors, then we can also color any geographical map with those same colors.
It's crucial to note that this theorem doesn't tell us how to find the right combination of colors for any particular map, it just assures us that it's possible with only four.
The proof for the theorem was discovered in the 1970s with the help of computer algorithms, causing some controversy because it wasn't a traditional mathematical proof that can be checked by humans.
In conclusion, the Four Color Theorem is a concept in graph theory stating that no more than four colors are needed to color the regions of a map so that no two adjacent regions have the same color. It is a unique aspect of mathematics where computer-aided proof has been used, and it is the explanation for why four colored pens are enough to fill in any map you come across, no matter how complicated it might be.

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