Ononot — The derangement Dn (permutations with no fixed points) satis

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Recursionhard

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The derangement $D_n$ (permutations with no fixed points) satisfies $D_n = (n-1)(D_{n-1} + D_{n-2})$ with $D_0 = 1$ and $D_1 = 0$ . What is $D_4$ ?