Seven Bridges of Königsberg

Graph theory

• https://en.wikipedia.org/wiki/Seven_Bridges_of_Königsberg
• MAA: Leonard Euler’s Solution to the Konigsberg Bridge Problem
• An historical note: Euler’s Königsberg letters

The island hosts Immanuel Kant‘s tomb.

Leonhard Euler first dismissed the problem

Thus you see, most noble Sir, how this type of solution to the Königsberg bridge problem bears little relationship to mathematics, and I do not understand why you expect a mathematician to produce it, rather than anyone else, for the solution is based on reason alone, and its discovery does not depend on any mathematical principle

but soon recognized the importance of the problem.

This question is so banal, but seemed to me worthy of attention in that neither geometry, nor algebra, nor even the art of counting [ars combinatoria] was sufficient to solve it. In view of this, it occurred to me to wonder whether it belonged to the geometry of position [geometria situs], which Leibniz had once so much longed for.