Primes Product, or Sum??

Let P P be the set of all odd primes, find the value of p P p 2 p 2 1 \prod_{p\in P}\dfrac{p^2}{p^2-1} .


The answer is 1.23370055.

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2 solutions

Chew-Seong Cheong
Aug 21, 2018

Relevant wiki: Prime Zeta Function

P = k = 2 p k 2 p k 2 1 where p k is the k th prime. = 2 2 1 2 2 k = 1 p k 2 p k 2 1 = 3 4 k = 1 1 1 p k 2 By Euler product ζ ( s ) = k = 1 ( 1 1 p k s ) 1 = 3 4 ζ ( 2 ) where ζ ( ) is the Riemann zeta function. = 3 4 ( π 2 6 ) see reference: P r i m e Z e t a F u n c t i o n . = π 2 8 1.234 \begin{aligned} P & = \prod_{\color{#3D99F6}k=2}^\infty \frac {p_k^2}{p_k^2-1} & \small \color{#3D99F6} \text{where }p_k \text{ is the }k\text{th prime.} \\ & = \frac {2^2-1}{2^2} \prod_{\color{#D61F06}k=1}^\infty \frac {p_k^2}{p_k^2-1} \\ & = \frac 34 \color{#3D99F6} \prod_{k=1}^\infty \frac 1{1- p_k^{-2}} & \small \color{#3D99F6} \text{By Euler product }\zeta (s) = \prod_{k=1}^\infty \left(1-\frac 1{p_k^s}\right)^{-1} \\ & = \frac 34 \color{#3D99F6} \zeta (2) & \small \color{#3D99F6} \text{where }\zeta (\cdot) \text{ is the Riemann zeta function.} \\ & = \frac 34 \left({\color{#3D99F6} \frac {\pi^2}6}\right) & \small \color{#3D99F6} \text{see reference: } Prime\ Zeta\ Function. \\ & = \frac {\pi^2}8 \approx \boxed{1.234} \end{aligned}

Thanks Sir, for providing external information to read!

Kelvin Hong - 2 years, 9 months ago
Kelvin Hong
Aug 20, 2018

p P p 2 p 2 1 = p P 1 1 1 / p 2 = ( 1 + 1 9 + 1 81 + ) ( 1 + 1 25 + 1 625 + ) . . . = 1 + 1 3 2 + 1 5 2 + 1 7 2 + 1 9 2 + 1 1 1 2 + = ζ ( 2 ) 1 4 ζ ( 2 ) = 3 4 π 2 6 = π 2 8 \begin{aligned}\prod_{p\in P}\dfrac{p^2}{p^2-1}&=\prod_{p\in P}\dfrac{1}{1-1/p^2}\\&=\bigg(1+\frac19+\frac1{81}+\dots\bigg)\bigg(1+\dfrac1{25}+\dfrac1{625}+\dots\bigg)...\\&=1+\dfrac1{3^2}+\dfrac1{5^2}+\dfrac1{7^2}+\dfrac1{9^2}+\dfrac1{11^2}+\dots\\&=\zeta(2)-\dfrac14\zeta(2)\\&=\dfrac34\cdot\dfrac{\pi^2}6\\&=\boxed{\dfrac{\pi^2}{8}}\end{aligned}

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