21.2: Definition
We often want to count how many ways we can “mix up” the objects in a collection.
a bijection from a finite set to itself
Once you have written the elements of a finite set in some order, think of a permutation as a way of re-ordering them.
Figure \(\PageIndex{1}\) contains tables of values for all six possible permutations of the set \(A = \{a,b,c\}\text{.}\) We have grouped them according to: all elements fixed; one element fixed and two mixed; all elements mixed.