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A Permutation of $n$ ordered objects in which none of the objects appears in its natural place. The function giving this quantity is the Subfactorial $!n$, defined by

!n \equiv n!\sum_{k=0}^n {(-1)^k\over k!}
\end{displaymath} (1)

!n\equiv\left[{n!\over e}\right],
\end{displaymath} (2)

where $k!$ is the usual Factorial and $[x]$ is the Nint function. These are also called Rencontres Numbers (named after rencontres solitaire), or Complete Permutations, or derangements. The number of derangements $!n=d(n)$ of length $n$ satisfy the Recurrence Relations
\end{displaymath} (3)

\end{displaymath} (4)

with $d(1)=0$ and $d(2)=1$. The first few are 0, 1, 2, 9, 44, 265, 1854, ... (Sloane's A000166). This sequence cannot be expressed as a fixed number of hypergeometric terms (Petkovsek et al. 1996, pp. 157-160).

See also Married Couples Problem, Permutation, Root, Subfactorial


Aitken, A. C. Determinants and Matrices. Westport, CT: Greenwood Pub., p. 135, 1983.

Ball, W. W. R. and Coxeter, H. S. M. Mathematical Recreations and Essays, 13th ed. New York: Dover, pp. 46-47, 1987.

Coolidge, J. L. An Introduction to Mathematical Probability. Oxford, England: Oxford University Press, p. 24, 1925.

Courant, R. and Robbins, H. What is Mathematics?: An Elementary Approach to Ideas and Methods, 2nd ed. Oxford, England: Oxford University Press, pp. 115-116, 1996.

de Montmort, P. R. Essai d'analyse sur les jeux de hasard. Paris, p. 132, 1713.

Dickau, R. M. ``Derangements.''

Durell, C. V. and Robson, A. Advanced Algebra. London, p. 459, 1937.

Petkovsek, M.; Wilf, H. S.; and Zeilberger, D. A=B. Wellesley, MA: A. K. Peters, 1996.

Roberts, F. S. Applied Combinatorics. Englewood Cliffs, NJ: Prentice-Hall, 1984.

Ruskey, F. ``Information on Derangements.''

Sloane, N. J. A. Sequence A000166/M1937 in ``An On-Line Version of the Encyclopedia of Integer Sequences.'' and Sloane, N. J. A. and Plouffe, S. The Encyclopedia of Integer Sequences. San Diego: Academic Press, 1995.

Stanley, R. P. Enumerative Combinatorics, Vol. 1. New York: Cambridge University Press, p. 67, 1986.

Vardi, I. Computational Recreations in Mathematica. Reading, MA: Addison-Wesley, p. 123, 1991.

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© 1996-9 Eric W. Weisstein