Stirling's formula derived simply

Abstract

Stirling's formula, the asymptotic expansion of $n!$ for $n$ large, or of $\Gamma (z)$ for $z\to \infty$, is derived directly from the recursion equation $\Gamma (z+1)=z\Gamma (s)$ and the normalization condition $\Gamma (1/2)=\sqrt{\pi }$.

4 pages; omitted second author added