Mertens' Third Theorem

Theorem

 * $\displaystyle \lim_{x \mathop \to \infty} \ln x \prod_{\substack {p \mathop \le x \\ \text {$p$ prime} } } \paren {1 - \dfrac 1 p} = e^{-\gamma}$

where $\gamma$ denotes the Euler-Mascheroni constant.