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Picture of the month
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/91/Petersen1_tiny.svg/360px-Petersen1_tiny.svg.png)
Credit: Leshabirukov
The Petersen graph, an undirected graph with 10 vertices and 15 edges, serves as a useful example or counterexample for many problems in graph theory.
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