The Four-Colorability of the World: Addendum on Planarity

After presenting my conclusions in my previous post on the four-colorability of the world to several friends and friends of friends, we pondered the question of whether the map of the world’s countries is planar. It is in fact not: consider the induced subgraph of (that is, consider the connections among) Turkey, Georgia, Armenia, Azerbaijan, Iran, Turkmenistan, Kazakhstan, and Russia.

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