# Factorial Notation

$n!$ is the product of the first $n$ positive integers for $n\ge 1$. $$n!=n(n-1)(n-2)(n-3) \cdots \times 3 \times 2 \times 1$$ $n!$ is read "$n$ factorial". For example, the product $5 \times 4 \times 3 \times 2 \times 1$ can be written as $5!$. Notice that $5 \times 4 \times 3$ can be written using [...]