# How to find the number of perfect matchings in complete graphs?

In wikipedia FKT algorithm is given for planar graphs. Not anything for complete graphs. I need to find the number of perfect matchings in complete graph of six vertices.

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It's just the number of ways of partitioning the six vertices into three sets of two vertices each, right? So that's 15; vertex 1 can go with any of the 5 others, then choose one of the 4 remaining, it can go with any of three others, then there are no more choices to make. $5\times3=15$.
Side note. For a general complete graph on $2n$ vertices, this number comes out to be $\frac{(2n)!}{n! 2^{n}}$. –  Srivatsan Aug 31 '11 at 11:19
Or even for $\#V = 2n$ we have $(2n - 1)!!$ perfect matchings. –  Aurélien Ooms May 17 at 10:22