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Topic: a gift exchange
Replies: 6   Last Post: Aug 10, 2013 11:20 PM

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Mark Rickert

Posts: 28
From: California
Registered: 8/10/13
Re: a gift exchange
Posted: Aug 10, 2013 11:20 PM
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Seems to me the answer is !(n-1) / [!(n-1) + !n]

!n are all of the permutations where nobody gets their own gift (derangements, subfactorial).
!(n-1) are all of the permutations where the first n-1 don't get their own gift, but the nth person does.



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