permutation
<mathematics> 1. An ordering of a certain number of elements of a given
set.
For instance, the permutations of (1,2,3) are (1,2,3) (2,3,1) (3,1,2) (3,2,1)
(1,3,2) (2,1,3).
Permutations form one of the canonical examples of a "group"  they can be
composed and you can find an inverse permutation that reverses the action of any
given permutation.
The number of permutations of r things taken from a set of n is
n P r = n! / (nr)!
where "n P r" is usually written with n and r as subscripts and n! is the
factorial of n.
What the football pools call a "permutation" is not a permutation but a
combination  the order does not matter.
2. A bijection for which the domain and range are the same set and so
f(f'(x)) = f'(f(x)) = x.
(20010510)
Nearby terms:
Permanent Virtual Circuit « Permanent Virtual
Connection « permission « permutation » \perp
» perplexity » persistence
