The number of "arrangements" in an ordering of 
items is given by either a combination
(order is ignored) or a permutation (order is significant).
An ordering (or order) is also a method for choosing the order in which elements are placed (i.e., a sorting function).
The Wolfram Language function Ordering[p]
gives the inverse permutation of a given permutation 
.
See also
Arrangement,
Combination,
Cutting,
Derangement,
Inverse Permutation,
Lexicographic
Order,
Monomial Order,
Ordering
Axioms,
Partial Order,
Permutation,
Sorting,
Total Order,
Transposition Order,
Well
Ordered Set
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Cite this as:
Weisstein, Eric W. "Ordering." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Ordering.html
Subject classifications
