The map shows the four colour theorem in practice.
The theorm states that:
… given any separation of a plane into contiguous regions, producing a figure called a map, no more than four colors are required to color the regions of the map so that no two adjacent regions have the same color. Adjacent means that two regions share a common boundary curve segment, not merely a corner where three or more regions meet.
In actual, fact the map uses 5 colours, if you include the white used for the oceans, although it would still technically be possible to draw a world map using only 4 colours, if landlocked countries shared the same colour as the ocean.
Want to read more on the subject? The good news is that there’s a surprising number of books about it:
- Four Colors Suffice: How the Map Problem Was Solved
- The Four-Color Theorem: History, Topological Foundations, and Idea of Proof
- The Four Colour Theorem
- Graphs, Colourings and the Four-Colour Theorem
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