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Let $K_n$ be the complete graph with $n$ vertices, where the vertices are labelled $1,2,3,\dots,n$. How many paths are there between $v_1$ and $v_n$ that the labels on the path are strictly increasing?

I know that I need to first choose the possible vertices in the path, then consider the possible order of those vertices.

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    What is the source? The timing suggests a contest or test.2012-04-23
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    The timestamp appears to have come from the same question posted here: http://answers.yahoo.com/question/index?qid=20120422204450AAUDze92012-04-23

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