An estimate for the factorial function, named after the Scottish mathematician James Stirling (1692–1770), though it was known earlier to De Moivre. It states that n! and $\sqrt{2\pi n}{\left(n/e\right)}^n$ are asymptotically equal. The estimate is always an underestimate, but for n = 100 is 99.9% accurate. Bounds for n! exist such as