Definition:Euler Product

Theorem
Let $\displaystyle f(s) = \sum_{n \in \N} a_n n^{-s}$ be a Dirichlet series, absolutely convergent on $\Re(s) > \sigma_a$ (see Abscissa of Absolute Convergence).


 * $\displaystyle \sum_{n=1}^\infty a_n n^{-s} = \prod_p \frac 1 {1-f(p)p^{-s}} $

for all $s$ with $\Re(s) > \sigma_a$, where $p$ ranges over the primes.

This representation for $f$ is called an Euler product for the Dirichlet series.

Proof
This is immediate from Product Form of Sum on Completely Multiplicative Function.