Euler, a great mathematician, turned it into a few problems that were brushed aside.

The seven lines in the picture above represent seven bridges, and the red dots represent the points where they intersect. Euler found that only when the pen reaches the intersection along one arc and then leaves along another arc can a pen be completed, that is, the arcs intersecting at these points are paired. Such intersections are called even points. If the arcs intersecting at these points are not paired, that is, there are odd arcs, then a stroke cannot be realized. Such a point is also called a "singularity".

Through analysis, Euler draws the following conclusions: if a one-stroke figure has only two singularities, that is, only one starting point and one ending point, then the figure drawn by one stroke is open; Either there is no singularity, or the end point is connected with the starting point, so that the figure drawn one by one is closed. Because the problem of seven bridges has four singularities, it is impossible to find a route through seven bridges, but each bridge only takes one trip. The famous "Seven Bridges of Konigsberg" was thus solved by Euler.