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Possible Duplicate:
What's the General Expression For Probability of a Failed Gift Exchange Draw

4 couples are doing a gift exchange. Names are drawn randomly. Everyone gets a gift, but no one is allowed to pick either themself or their spouse. How many successful arrangements are there?

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    I think one of the answers at http://math.stackexchange.com/questions/85470/secret-santa-problem covers this.2011-12-12
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    See also http://math.stackexchange.com/questions/73341/whats-the-general-expression-for-probability-of-a-failed-gift-exchange-draw2011-12-12
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    [This answer](http://math.stackexchange.com/a/85559/12042) gives the numerical value as $4752$; [this one](http://math.stackexchange.com/a/73364/12042) gives the general formula.2011-12-12
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    Is there a reason why neither of you voted to close as duplicate?2011-12-12
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    [Here's a link](http://www.wolframalpha.com/input/?i=integral+of+%28x%5E2-4x%2B2%29%5E4e%5E%28-x%29+for+x%3D0..infinity) for the computation according to the general formula Brian linked to. I'm voting to close as a duplicate of that question.2011-12-12
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    @joriki, speaking only for myself, I was too rushed to read the other questions closely enough to be certain there is a duplication. But if you are convinced, pick one, vote to close, and I'll go along with you.2011-12-12

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