The Königsberg (now Kaliningrad) Cathedral sits on an island in the River Pregel. In 1736 Leonard Euler (pronounced ‘oiler’) resolved a question then being discussed as to whether it was possible to take a walk in Königsberg in such a way that each of the seven bridges in the town would be crossed once and only once, and the walker return to his starting point. This has become known as ‘the Königsberg bridge problem’. Mathematically it can be turned into a problem in graph theory. It has an easy solution – if you know how to do it!
