# Factorials; Permutations; Homework for 2014-09-08

Factorial: $n!=n\times(n-1)\times\ldots\times2\times1$
Number of permutations of $n$ distinct objects: $_nP_n = P(n,n) = n!$
Number of arrangements of $r$ distinct objects from a group of $n$ distinct objects: $_nP_r = P(n,r) = \frac{n!}{(n-r)!}$