the sum n = 1 ∑ ∞ n 2 τ ( n ) = 3 6 π 4 This has a very simple closed form, and is easy to work out.
However, the sum: n = 1 ∑ ∞ n 2 ln ( n ) τ ( n ) Has a very difficult closed form, of the form − b π a ( c γ + ln ( d π ) + f ln ( A ) ) Find a + b + c + d + f .
Notations:
All of a , b , c , d , f are integers.
τ ( n ) means the number of positive divisors of n.
A is the Glaisher-Kinkelin Constant
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Nice approach used.
... i put 18 as 108 in my calculator first, so i had to add 90 to the problem.
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This problem is unnecessarily complicated though, I did not know of a close form for ζ ′ ( 2 ) until last night at 0000 when I researched for it.
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i learned it 5 days ago
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@Aareyan Manzoor – I learned it when I was trying to determine if this sum was interesting ;)
Is there a sign error in your equation 2 ζ ( s ) ζ ′ ( s ) = . . . ? It seems to me that the LHS is negative while the RHS is positive.
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That appears to be the case, did i do something wrong.
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Just a sign error... the answer is the negative of what you found.
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@Otto Bretscher – right, I found my mistake, thanks for pointing it out.
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@Aareyan Manzoor – It's funny that two other people got it "right" anyways ;)
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We have ζ 2 ( s ) = n = 1 ∑ ∞ n s τ ( n ) This is easily found by dirichlet series . d.w.r.s 2 ζ ( s ) ζ ′ ( s ) = − n = 1 ∑ ∞ n s ln ( n ) τ ( n ) put s=2 n = 1 ∑ ∞ n 2 ln ( n ) τ ( n ) = − 2 ζ ( 2 ) ζ ′ ( 2 ) The popular closed form of ζ ′ ( 2 ) is 6 π 2 ( γ + ln ( 2 π ) − 1 2 ln ( A ) ) Putting that in we get − 1 8 π 4 ( 1 γ + ln ( 2 π ) − 1 2 ln ( A ) ) and 4 + 1 8 + 1 + 2 − 1 2 = 1 3