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Aderangement(orcomplete permutation) of a set is a permutation that leaves no element in its original position. The number of derangements of a set withnelements can be computed recursively using this formula:

Using the principle of inclusion and exclusion, we also get:

Here are some diagrams that represent the derangements of sets with

nelements.3 elements, 2 derangements:

4 elements, 9 derangements:

5 elements, 44 derangements:

6 elements, 265 derangements:

(Diagram is several dozen kilobytes, so let me know if you're really intent on seeing it.)Derangement formulas from Fred S. Roberts,

Applied Combinatorics, Prentice-Hall, 1984.Designed and rendered on a Saturday using

Mathematica3.0 for the Apple Macintosh.

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