Singularity can be used to judge whether a figure can be drawn with one stroke. The necessary condition of a stroke graph is that the number of singularities is 0 or 2, that is, when the lines of a graph are connected and the number of odd points is 0 or 2, the graph can be drawn with one stroke.
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/kloc-In the 8th century, in a park in Konigsberg, there were seven bridges connecting two islands in the Fritz fritz pregl River and their banks. Can we start from any of these four places, cross each bridge only once, and then go back to the starting point?
After the problem of the Seven Bridges was put forward, many people were interested in it and tried it one after another, but for a long time, it was never solved, thus forming the famous "Seven Bridges in Konigsberg".
1735, several college students wrote to Euler, a talented mathematician who worked at the Academy of Sciences in Petersburg, Russia, and asked him to help solve this problem. After personally observing the seventh bridge in Konigsberg, Euler seriously thought about the way to go, but he never succeeded.
After a year's research, 1736, 29-year-old Euler submitted his paper "Seven Bridges in Konigsberg" to St. Petersburg Academy of Sciences. While answering questions, he founded a new branch of mathematics-graph theory and geometric topology, and thus launched a new course in the history of mathematics.
Through the study of the Seven Bridges problem, Euler not only answered this question satisfactorily, but also obtained and proved three more extensive conclusions about a sum, which people usually call "euler theorem F".
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