Stirling's Formula

Theorem
The factorial function can be approximated by the formula:
 * $n! \sim \sqrt {2 \pi n} \left({\dfrac n e}\right)^n$

where $\sim$ denotes asymptotically equal.

Also defined as
This result can also be seen reported as:
 * $n! \sim \sqrt {2 \pi} n^n n^{1/2} e^{-n}$

Other variants are sometimes encountered.

Also known as
It is otherwise known as Stirling's approximation.

Also see

 * Limit of Error in Stirling's Formula