Euler Product

For $\sigma>1$,

$$\zeta(\sigma)\equiv \sum_{n=1}^\infty {1\over n^\sigma} = \prod_p {1\over 1-{1\over p^\sigma}},$$

where $\zeta(z)$ is the Riemann Zeta Function.

$$e^\gamma=\lim_{n\to\infty} {1\over\ln n} \prod_{i=1}^n {1\over 1-{1\over p_i}},$$

where the product is over Primes $p$, where $\gamma$ is the Euler-Mascheroni Constant.

See also Dedekind Function