While reading Perec's "Species of Spaces" I come across with Königsberg problem. Now I have another reason to visit Kaliningrad !
The Seven Bridges of Königsberg is a notable historical problem in mathematics. Its negative resolution by Leonhard Euler in 1735 laid the foundations of graph theory and prefigured the idea oftopology.
The city of Königsberg in Prussia (now Kaliningrad, Russia) was set on both sides of the Pregel River, and included two large islands which were connected to each other and the mainland by seven bridges.
The problem was to find a walk through the city that would cross each bridge once and only once. The islands could not be reached by any route other than the bridges, and every bridge must have been crossed completely every time (one could not walk half way onto the bridge and then turn around and later cross the other half from the other side).
In the second picture, greens are the bridges that remain from the original 7 and the red ones are the ones that were demolished.
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