Figure 1.

The “Königsberg bridge problem” as a graph. (A) In Euler’s time, one part of Königsberg (a) was connected with two bridges to the smaller isle (b) in the Pregel River and with one bridge to the larger isle (d). The same was true for the other part of the city (c). The problem was to create a path that would cross each bridge only once. (B) Euler solved the problem by representing the system as a graph in which the isles were represented as nodes and the bridges as links between them. (C) Euler´s path is shown on the current map of Kaliningrad.