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I am trying to figure out how many permutations exist in a set where none of the numbers equal their own position in the set; for example, 3,1,5,2,4 is an acceptable permutation where 3,1,2,4,5 is not because 5 is in position 5. I know that the number of total permutations is n!. Is there a formula for how many are acceptable given the case that no position holds its own number?

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Check out en.wikipedia.org/wiki/Derangement –  Florian Dec 17 '10 at 13:13
    
Florian, your comment should be an answer. –  Anthony Labarre Dec 17 '10 at 13:15
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This recent math.SE question asks for a proof of one of the formulas for the number of derangements: math.stackexchange.com/questions/14477. –  Mike Spivey Dec 17 '10 at 15:52

3 Answers 3

What you are looking for is known as derangement. However, for counting the number of derangement for say $n$-elements you could possible use a trick, compute$\frac{n!}{e}$ and then round off to an integer and this will give you the desired result.

This is actually another application of $e$, which was discovered by Jacob Bernoulli in the problem of derangement, also known as the hat check problem.

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In his combinatorics lecture notes, Jacob Lurie analyses the problem of derangements. It is at the end of these note: http://www.math.harvard.edu/~lurie/155notes/lecture3.pdf

and continues into these

http://www.math.harvard.edu/~lurie/155notes/lecture4.pdf.

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Check out http://en.wikipedia.org/wiki/Derangement

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While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. –  Gamma Function Aug 5 at 23:27
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Please add more content from the cited page (e.g. describe derangements, etc). Otherwise, this should really be left as a comment. –  robjohn Aug 7 at 0:07

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