Random derangements and the Ewens Sampling Formula

We study derangements of $\{1,2,\ldots,n\}$ under the Ewens distribution with parameter $θ$. We give the moments and marginal distributions of the cycle counts, the number of cycles, and asymptotic distributions for large $n$. We develop a $\{0,1\}$-valued non-homogeneous Markov chain with the property that the counts of lengths of spacings between the 1s have the derangement distribution. This chain, an analog of the so-called Feller Coupling, provides a simple way to simulate derangements in time independent of $θ$ for a given $n$ and linear in the size of the derangement.

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