What famous problem to Leonhard Euler solve in 1736?

Answer 1

Euler solved the Königsberg bridge problem in 1736. Here is a quote from Euler about the problem. "A problem was posed to me about an island in the city of Königsberg, surrounded by a river spanned by seven bridges, and I was asked whether someone could traverse the separate bridges in a connected walk in such a way that each bridge is crossed only once.....This question is so banal, but seemed to be worthy of attention in that geometry, nor algebra, nor even the art of counting was sufficient to solve it. In view of this, it occurred to me to wonder whether it belonged to the geometry of position which Leibniz had once so much longed for. And so, after some deliberation, I obtained a simple, yet completely established, rule with whose help one can immediately decided for all examples of this kind, with any number of bridges in any arrangement ...." Leonhard Euler to Giovanni Marinoni March 13, 1736 With current graph theory, this problem is quite straightforward. But Euler did not have that information so his solution was remarkable!