Euler product

If $f$ is a multiplicative function, then

\sum_{n=1}^{\infty}f(n)=\prod_{p\text{ is prime}}(1+f(p)+f(p^{2})+\cdots)

(1)

provided the sum on the left converges absolutely. The product on the right is called the Euler product for the sum on the left.

Proof of (1).

Expand partial products on right of (1) to obtain by fundamental theorem of arithmetic

	$\displaystyle\prod_{p<y}(1+f(p)+f(p^{2})+\cdots)$	$\displaystyle=\sum_{k_{1}}f(p_{1}^{k_{1}})\sum_{k_{2}}f(p_{2}^{k_{2}})\cdots% \sum_{k_{t}}f(p_{t}^{k_{t}})$
		$\displaystyle=\sum_{k_{1},k_{2},\ldots,k_{t}}f(p_{1}^{k_{1}})f(p_{2}^{k_{2}})% \cdots f(p_{t}^{k_{t}})$
		$\displaystyle=\sum_{k_{1},k_{2},\ldots,k_{t}}f(p_{1}^{k_{1}}p_{2}^{k_{2}}% \cdots p_{t}^{k_{t}})$
		$\displaystyle=\sum_{P_{+}(n)<y}f(n)$

where $p_{1},p_{2},\ldots,p_{t}$ are all the primes between $1$ and $y$ , and $P_{+}(n)$ denotes the largest prime factor of $n$ . Since every natural number less than $y$ has no factors exceeding $y$ we have that

\left\lvert\sum_{n=1}^{\infty}f(n)-\sum_{P_{+}(n)<y}f(n)\right\rvert\leq\sum_{% n=y}^{\infty}\left\lvert f(n)\right\rvert

which tends to zero as $y\to\infty$ . ∎

Examples

•

If the function $f$ is defined on prime powers by $f(p^{k})=1/p^{k}$ for all $p<x$ and $f(p^{k})=0$ for all $p\geq x$ , then allows one to estimate $\prod_{p<x}(1+1/(p-1))$

$\prod_{p<x}\left(1+\frac{1}{p-1}\right)=\prod_{p<x}\left(1+\frac{1}{p}+\frac{1% }{p^{2}}+\cdots\right)=\sum_{P_{+}(n)<x}\frac{1}{n}>\sum_{n<x}\frac{1}{n}>\ln x.$

One of the consequences of this formula is that there are infinitely many primes.
•

The Riemann zeta function is defined by the means of the series

$\zeta(s)=\sum_{n=1}^{\infty}n^{-s}\qquad\text{for $\Re s>1$}.$

Since the series converges absolutely, the Euler product for the zeta function is

$\zeta(s)=\prod_{p}\frac{1}{1-p^{-s}}\qquad\text{for $\Re s>1$}.$

If we set $s=2$ , then on the one hand $\zeta(s)=\sum_{n}1/n^{2}$ is $\pi^{2}/6$ (proof is here (http://planetmath.org/ValueOfTheRiemannZetaFunctionAtS2)), an irrational number, and on the other hand $\zeta(2)$ is a product of rational functions of primes. This yields yet another proof of infinitude of primes.

Title	Euler product
Canonical name	EulerProduct
Date of creation	2013-03-22 14:10:58
Last modified on	2013-03-22 14:10:58
Owner	bbukh (348)
Last modified by	bbukh (348)
Numerical id	8
Author	bbukh (348)
Entry type	Definition
Classification	msc 11A05
Classification	msc 11A51
Related topic	MultiplicativeFunction
Related topic	RiemannZetaFunction