Back in the 1700s, a city known as Königsberg, in what is now Russia, consisted of two islands on the Pregel River and the surrounding land. The city wasn’t known for anything in particular, or really anything at all. But because of some interesting geography and two creative mathematicians, Königsberg has gone down in the history of mathematics as a key city in the development of a whole new genre of mathematics: Graph Theory.
The story starts with a mathematician named Carl Gottlieb Ehler. Ehler noticed that the islands of Königsberg only had seven bridges going from the mainland to the islands and one bridge going from island to island. This caused Ehler to wonder if there was a way to make a trip such that you traveled across each bridge only once. The great video above, made by TED-Ed, explains how Ehler, with the help of the mathematician Leonhard Euler, came to a conclusion to the “Königsberg Bridge Problem” and at the same time invented a whole new type of math.