The factorial of a number n written n! is the product of n along with all the natural numbers smaller than n (not including zero !!!).

Therefore, n! = n x (n - 1) x (n - 2) ... 3 x 2 x 1

For example, 5! = 5 x 4 x 3 x 2 x 1 = 120

Here are some facts about the factorial:

• Stirling's formula gives an approximation of the factorial:
• $n!{\approx}n^ne^{-n}\sqrt{2\pi{n}}$
• There is also a version of the factorial that is defined for (some) non-integers:
• $n!=\int_0^\infty{e^{-x}}x^n.dx$
See the Gamma Function