Permutation with Repetition

Permutation with Repetition is an arrangement of $r$ items chosen from $n$ distinct types, where order matters and each type can be used more than once.

Since each of the $r$ positions can be filled in $n$ ways, the number of permutations with repetition is $n^r$. For example, a 4-digit PIN using the digits 0-9 has $10^4 = 10{,}000$ possibilities.

See Permutation and R-Permutation.