2009 OIM Problems/Problem 1

Problem

Let $n$ be a natural number greater than 2. Suppose that $n$ islands are located in a circle and that between each two neighboring islands there are two bridges, with the islands $x_1, x_2, \cdots , x_n$ in order of the clock hands. Starting on island $x_1$ , in how many ways can the $2n$ bridges be crossed by passing over each bridge exactly once?

~translated into English by Tomas Diaz. ~orders@tomasdiaz.com

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2009 OIM Problems/Problem 1

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