Definition:Euler Product

Definition
Let $a_n : \N \to \C$ be an arithmetic function.

Let $\ds \map f s = \sum_{n \mathop \in \N} a_n n^{-s}$ be its Dirichlet series.

Let $\sigma_a$ be its abscissa of absolute convergence.

From Product Form of Sum on Completely Multiplicative Function, for $\map \Re s > \sigma_a$ we have:
 * $\ds \sum_{n \mathop = 1}^\infty a_n n^{-s} = \prod_p \frac 1 {1 - a_p p^{-s} }$

where $p$ ranges over the primes.

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

Also see

 * Product Form of Sum on Completely Multiplicative Function