A = ⎣ ⎡ p = 6 k + 1 is prime, p > 3 ∏ ( 1 + p − 1 ) ⎦ ⎤ ⎣ ⎡ p = 6 k − 1 is prime, p > 3 ∏ ( 1 − p − 1 ) ⎦ ⎤
Find the value of ⌊ 1 0 0 0 0 0 A ⌋ .
Clue : Use the Legendre symbol and the fact that it is completely multiplicative.
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As pointed out by Patrick, more accurately what you are doing is taking the Euler product form of the Dirichlet L-function. If χ satisfies χ ( m n ) = χ ( m ) × χ ( n ) for coprime integers m , n , when we have
primes p ∏ ( 1 − p s χ ( p ) = ( n = 1 ∑ ∞ n 2 χ ( n ) ) − 1
Unfortunately, this equation is only valid for R e ( s ) > 1 , whereas you want to apply 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 ) as the Dirichlet character ( p − 3 ) 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
How did you get the first line and the second last line (of equations)?
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For the first line: Through the Legendre symbol.
For the last line:
n = 0 ∑ ∞ ( 3 n + 1 1 − 3 n + 2 1 ) = n = 0 ∑ ∞ ∫ 0 1 x 3 n + x 3 n + 1 d x
= ∫ 0 1 − x 3 − 1 x + 1 d x = 3 3 π
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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?
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@Pi Han Goh – Every prime bigger than 3 can be expressed as 6 k + 1 or 6 k − 1 , where k is some integer. So in the left side of the equation, all the primes bigger than 3 are involved.
Since X ( n ) = { − 1 if n = − 1 mod ( 6 ) 1 if n = 1 mod ( 6 )
The left side can be combined into the right side.
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@Julian Poon – Ohhhh got it! Why I didn't see that?!? Ahaha THANKS
@Jake Lai FYI!
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 . 8 2 7 2 7 1 7 7 3 0 9 5 . Thanks for the information, I'll try to improve the solution after understanding what you meant.
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⎣ ⎡ 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 ) Where X ( n ) = ( 3 n ) which is the Legendre symbol.
Since 1 + X ( p ) p − s = p s X ( p ) + p s = p s X ( p ) + 1 = p s X ( p ) − 1 p 2 s 1 − 1 = 1 − p s X ( p ) 1 − p 2 s 1
p is prime, p > 3 ∏ ( 1 + X ( p ) p − s ) = ∏ p is prime, p > 3 ( 1 − p s X ( p ) ) ∏ p is prime, p > 3 ( 1 − p 2 s 1 )
Now, through Euler's product, ∏ p is prime ( 1 − p 2 s 1 ) = ζ ( 2 s ) 1 and ∏ p is prime ( 1 − p s X ( p ) ) = ∑ n = 1 ∞ n s X ( n ) 1 , where ζ ( s ) is the Riemann zeta function.
So, ∏ p is prime, p > 3 ( 1 − p s X ( p ) ) ∏ p is prime, p > 3 ( 1 − p 2 s 1 ) = ( 6 2 s − 2 2 s − 3 2 s + 1 ) ζ ( 2 s ) 6 2 s ( 1 + 2 s 1 ) n = 1 ∑ ∞ n s X ( n )
Since X ( n ) = ⎩ ⎪ ⎨ ⎪ ⎧ 0 if n ≡ 0 mod ( 3 ) − 1 if n ≡ − 1 mod ( 3 ) 1 if n ≡ 1 mod ( 3 )
n = 1 ∑ ∞ n s X ( n ) = n = 0 ∑ ∞ ( ( 3 n + 1 ) s 1 − ( 3 n + 2 ) s 1 )
Therefore, ∏ p is prime, p > 3 ( 1 − p s X ( p ) ) ∏ p is prime, p > 3 ( 1 − p 2 s 1 ) = ( 6 2 s − 2 2 s − 3 2 s + 1 ) ζ ( 2 s ) 6 2 s ( 1 + 2 s 1 ) n = 0 ∑ ∞ ( ( 3 n + 1 ) s 1 − ( 3 n + 2 ) s 1 )
Substituting s = 1 , we get: 4 π 2 5 4 n = 0 ∑ ∞ ( 3 n + 1 1 − 3 n + 2 1 ) = 4 π 2 5 4 n = 0 ∑ ∞ ∫ 0 1 x 3 n + x 3 n + 1 d x
= 4 π 2 5 4 3 3 π = 2 π 3 3 = A
And hence ⌊ 1 0 0 0 0 0 A ⌋ = 8 2 6 9 9