Permutations
A permutation is an arrangement of objects without repetition where order is important.
Permutations using all the objects
A permutation of n objects, arranged into one group of size n, without repetition, and order being important is:
nPn = P(n,n) = n!
Example: Find all permutations of the letters "ABC"
ABC ACB BAC BCA CAB CBA
Permutations of some of the objects
A permutation of n objects, arranged in groups of size r, without repetition, and order being important is:
nPr = P(n,r) = n! / (n-r)!
Example: Find all two-letter permutations of the letters "ABC"
AB AC BA BC CA CB
Shortcut formula for finding a permutation
Assuming that you start a n and count down to 1 in your factorials ...
P(n,r) = first r factors of n factorial
