# Permutations

Arrangement of r objects from a set of n objects, where \(r \leq n\).

E.g. for “abc” we have abc acb bac bca cab cba

# Formula

## Base

nPr = n * (n - 1) * (n - 2) * … * (n - (r - 1)) = n!/(n-r)!

## Duplicates

e.g. abccdd

nPr / 2!2!

## Circle

n! / n = (n - 1)!