Jake Lai's Problem

A = [ p = 6 k + 1 is prime, p > 3 ( 1 + p 1 ) ] [ p = 6 k 1 is prime, p > 3 ( 1 p 1 ) ] A=\left[ \prod _{ p=6k+1 \text{ is prime, } p>3 } (1+p^{ -1 }) \right] \left[ \prod _{ p=6k-1 \text{ is prime, }p>3 } (1-p^{ -1 }) \right]

Find the value of 100000 A \left\lfloor 100000A \right\rfloor .

Clue : Use the Legendre symbol and the fact that it is completely multiplicative.


My examinations are finally over! And I have time to re-look at one of Jake Lai's problems, which has unfortunately been deleted. So here I'm just going to re-post it.


The answer is 82699.

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1 solution

Julian Poon
Oct 6, 2015

[ p = 6 k + 1 is prime, p > 3 ( 1 + p s ) ] [ p = 6 k 1 is prime, p > 3 ( 1 p s ) ] = p is prime, p > 3 ( 1 + X ( p ) p s ) \left[ \prod _{ p=6k+1 \text{ is prime, } p>3 } (1+p^{ -s }) \right] \left[ \prod _{ p=6k-1 \text{ is prime, }p>3 } (1-p^{ -s }) \right] = \prod _{ p \text{ is prime, }p>3 } (1+X(p)p^{ -s }) Where X ( n ) = ( n 3 ) X(n)=\left(\frac{n}{3}\right) which is the Legendre symbol.

Since 1 + X ( p ) p s = X ( p ) + p s p s = X ( p ) p s + 1 = 1 p 2 s 1 X ( p ) p s 1 = 1 1 p 2 s 1 X ( p ) p s 1+X(p)p^{ -s }=\frac { X\left( p \right) +p^{ s } }{ p^{ s } } =\frac { X\left( p \right) }{ p^{ s } } +1=\frac { \frac { 1 }{ p^{ 2s } } -1 }{ \frac { X\left( p \right) }{ p^{ s } } -1 } =\frac { 1-\frac { 1 }{ p^{ 2s } } }{ 1-\frac { X\left( p \right) }{ p^{ s } } }

p is prime, p > 3 ( 1 + X ( p ) p s ) = p is prime, p > 3 ( 1 1 p 2 s ) p is prime, p > 3 ( 1 X ( p ) p s ) \prod _{ p \text{ is prime, }p>3 } (1+X(p)p^{ -s }) = \frac { \prod _{ p \text{ is prime, }p>3 } \left( 1-\frac { 1 }{ p^{ 2s } } \right) }{ \prod _{ p \text{ is prime, }p>3 } \left( 1-\frac { X\left( p \right) }{ p^{ s } } \right) }

Now, through Euler's product, p is prime ( 1 1 p 2 s ) = 1 ζ ( 2 s ) \prod _{ p \text{ is prime} } \left( 1-\frac { 1 }{ p^{ 2s } } \right) =\frac { 1 }{ \zeta (2s) } and p is prime ( 1 X ( p ) p s ) = 1 n = 1 X ( n ) n s \prod _{ p \text{ is prime} } \left( 1-\frac { X\left( p \right) }{ p^{ s } } \right) =\frac { 1 }{ \sum _{ n=1 }^{ \infty } \frac { X\left( n \right) }{ n^{ s } } } , where ζ ( s ) \zeta (s) is the Riemann zeta function.

So, p is prime, p > 3 ( 1 1 p 2 s ) p is prime, p > 3 ( 1 X ( p ) p s ) = 6 2 s ( 6 2 s 2 2 s 3 2 s + 1 ) ζ ( 2 s ) ( 1 + 1 2 s ) n = 1 X ( n ) n s \frac { \prod _{ p \text{ is prime, }p>3 } \left( 1-\frac { 1 }{ p^{ 2s } } \right) }{ \prod _{ p \text{ is prime, }p>3 } \left( 1-\frac { X\left( p \right) }{ p^{ s } } \right) } = \frac { 6^{ 2s } }{ \left( 6^{ 2s }-2^{ 2s }-3^{ 2s }+1 \right) \zeta (2s) } \left( 1+\frac { 1 }{ 2^{ s } } \right) \sum _{ n=1 }^{ \infty } \frac { X\left( n \right) }{ n^{ s } }

Since X ( n ) = { 0 if n 0 mod ( 3 ) 1 if n 1 mod ( 3 ) 1 if n 1 mod ( 3 ) X\left( n \right) =\begin{cases} 0\text{ if }n\equiv 0\quad \text{mod}(3) \\ -1\text{ if }n\equiv -1\quad \text{mod}(3) \\ 1\text{ if }n\equiv 1\quad \text{mod}(3) \end{cases}

n = 1 X ( n ) n s = n = 0 ( 1 ( 3 n + 1 ) s 1 ( 3 n + 2 ) s ) \sum _{ n=1 }^{ \infty } \frac { X\left( n \right) }{ n^{ s } } =\sum _{ n=0 }^{ \infty } \left( \frac { 1 }{ \left( 3n+1 \right) ^{ s } } -\frac { 1 }{ \left( 3n+2 \right) ^{ s } } \right) \\

Therefore, p is prime, p > 3 ( 1 1 p 2 s ) p is prime, p > 3 ( 1 X ( p ) p s ) = 6 2 s ( 6 2 s 2 2 s 3 2 s + 1 ) ζ ( 2 s ) ( 1 + 1 2 s ) n = 0 ( 1 ( 3 n + 1 ) s 1 ( 3 n + 2 ) s ) \frac { \prod _{ p \text{ is prime, }p>3 } \left( 1-\frac { 1 }{ p^{ 2s } } \right) }{ \prod _{ p \text{ is prime, }p>3 } \left( 1-\frac { X\left( p \right) }{ p^{ s } } \right) }= \frac { 6^{ 2s } }{ \left( 6^{ 2s }-2^{ 2s }-3^{ 2s }+1 \right) \zeta (2s) } \left( 1+\frac { 1 }{ 2^{ s } } \right) \sum _{ n=0 }^{ \infty } \left( \frac { 1 }{ \left( 3n+1 \right) ^{ s } } -\frac { 1 }{ \left( 3n+2 \right) ^{ s } } \right)

Substituting s = 1 s=1 , we get: 54 4 π 2 n = 0 ( 1 3 n + 1 1 3 n + 2 ) = 54 4 π 2 n = 0 0 1 x 3 n + x 3 n + 1 d x \frac{54}{4\pi ^2} \sum _{ n=0 }^{ \infty } \left( \frac { 1 }{ 3n+1 } -\frac { 1 }{ 3n+2 } \right) =\frac { 54 }{ 4\pi ^{ 2 } } \sum _{ n=0 }^{ \infty } \int _{ 0 }^{ 1 }{ { x }^{ 3n }+ } { x }^{ 3n+1 }dx

= 54 4 π 2 π 3 3 = 3 3 2 π = A =\frac { 54 }{ 4\pi ^{ 2 } } \frac { \pi }{ 3\sqrt { 3 } } =\frac{3\sqrt{3}}{2\pi }=A

And hence 100000 A = 82699 \left\lfloor 100000A \right\rfloor =\boxed{82699}

Moderator note:

As pointed out by Patrick, more accurately what you are doing is taking the Euler product form of the Dirichlet L-function. If χ \chi satisfies χ ( m n ) = χ ( m ) × χ ( n ) \chi (mn ) = \chi (m) \times \chi (n) for coprime integers m , n m,n , when we have

primes p ( 1 χ ( p ) p s = ( n = 1 χ ( n ) n 2 ) 1 \prod_{\text{primes } p } ( 1 - \frac{ \chi (p)} { p^ s } = \left( \sum_{n=1} ^ \infty \frac{ \chi (n) } { n^2 } \right) ^{-1}

Unfortunately, this equation is only valid for R e ( s ) > 1 Re (s) > 1 , whereas you want to apply s = 1 s =1 . This L-function can be extended via analytic continuation to a meromorphic function (which requires work). Furthermore, you need to show that when you're using a non-principle characteristic function, then the infinite product converges to the infinite sum that we want.

My solution was similar, but I wrote X ( p ) X(p) as the Dirichlet character ( 3 p ) \left( \frac{-3}{p} \right) and then cheated by looking at a table of special values for Dirichlet L-functions:

http://www.people.fas.harvard.edu/~sfinch/csolve/ls.pdf

Patrick Corn - 5 years, 8 months ago

How did you get the first line and the second last line (of equations)?

Pi Han Goh - 5 years, 8 months ago

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For the first line: Through the Legendre symbol.

Source

For the last line:

n = 0 ( 1 3 n + 1 1 3 n + 2 ) = n = 0 0 1 x 3 n + x 3 n + 1 d x \sum _{ n=0 }^{ \infty } \left( \frac { 1 }{ 3n+1 } -\frac { 1 }{ 3n+2 } \right) = \sum _{ n=0 }^{ \infty } \int _{ 0 }^{ 1 }{ { x }^{ 3n }+ } { x }^{ 3n+1 }dx

= 0 1 x + 1 x 3 1 d x = π 3 3 =\int _{ 0 }^{ 1 }{ -\frac { x+1 }{ { x }^{ 3 }-1 } } dx=\frac { \pi }{ 3\sqrt { 3 } }

Julian Poon - 5 years, 8 months ago

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Oh I figured your second last line already (which I think could be elaborated). Anyway, in the 1st line, how does the product of 2 infinite products become and infinite product with X(n) in it?

Pi Han Goh - 5 years, 8 months ago

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@Pi Han Goh Every prime bigger than 3 3 can be expressed as 6 k + 1 6k+1 or 6 k 1 6k-1 , where k k is some integer. So in the left side of the equation, all the primes bigger than 3 3 are involved.

Since X ( n ) = { 1 if n = 1 mod ( 6 ) 1 if n = 1 mod ( 6 ) X(n)=\begin{cases} -1\text{ if }n=-1\text{ mod}(6) \\ 1\text{ if }n=1\text{ mod}(6) \end{cases}

The left side can be combined into the right side.

Julian Poon - 5 years, 8 months ago

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@Julian Poon Ohhhh got it! Why I didn't see that?!? Ahaha THANKS

Pi Han Goh - 5 years, 8 months ago

@Jake Lai FYI!

Calvin Lin Staff - 5 years, 8 months ago

Response to challenge master note:

I was quite surprised that the infinite product converges to the infinite sum, after calculating numerically to the the 10000th prime and getting the number 0.827271773095 0.827271773095 . Thanks for the information, I'll try to improve the solution after understanding what you meant.

Julian Poon - 5 years, 8 months ago

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