ln ( n ) \ln(n) can make a difference

Calculus Level 5

the sum n = 1 τ ( n ) n 2 = π 4 36 \sum_{n=1}^\infty \dfrac{\tau(n)}{n^2}=\dfrac{\pi^4}{36} This has a very simple closed form, and is easy to work out.

However, the sum: n = 1 ln ( n ) τ ( n ) n 2 \sum_{n=1}^\infty \dfrac{\ln(n)\tau(n)}{n^2} Has a very difficult closed form, of the form π a b ( c γ + ln ( d π ) + f ln ( A ) ) -\dfrac{\pi^a}{b}(c\gamma+\ln(d\pi)+f\ln(A)) Find a + b + c + d + f a+b+c+d+f .

Notations:

All of a , b , c , d , f a,b,c,d,f are integers.

τ ( n ) \tau(n) means the number of positive divisors of n.

A is the Glaisher-Kinkelin Constant


The answer is 13.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Aareyan Manzoor
Feb 18, 2016

We have ζ 2 ( s ) = n = 1 τ ( n ) n s \zeta^2(s)=\sum_{n=1}^\infty \dfrac{\tau(n)}{n^s} This is easily found by dirichlet series . d.w.r.s 2 ζ ( s ) ζ ( s ) = n = 1 ln ( n ) τ ( n ) n s 2\zeta(s)\zeta'(s)=-\sum_{n=1}^\infty \dfrac{\ln(n)\tau(n)}{n^s} put s=2 n = 1 ln ( n ) τ ( n ) n 2 = 2 ζ ( 2 ) ζ ( 2 ) \sum_{n=1}^\infty \dfrac{\ln(n)\tau(n)}{n^2}=-2\zeta(2)\zeta'(2) The popular closed form of ζ ( 2 ) \zeta'(2) is π 2 6 ( γ + ln ( 2 π ) 12 ln ( A ) ) \dfrac{\pi^2}{6} (\gamma+\ln(2\pi)-12\ln(A)) Putting that in we get π 4 18 ( 1 γ + ln ( 2 π ) 12 ln ( A ) ) -\dfrac{\pi^4}{18}(1\gamma+\ln(2\pi)-12\ln(A)) and 4 + 18 + 1 + 2 12 = 13 4+18+1+2-12=13

Moderator note:

Nice approach used.

... i put 18 as 108 in my calculator first, so i had to add 90 to the problem.

Aareyan Manzoor - 5 years, 3 months ago

Log in to reply

This problem is unnecessarily complicated though, I did not know of a close form for ζ ( 2 ) \zeta'(2) until last night at 0000 when I researched for it.

Julian Poon - 5 years, 3 months ago

Log in to reply

i learned it 5 days ago

Aareyan Manzoor - 5 years, 3 months ago

Log in to reply

@Aareyan Manzoor I learned it when I was trying to determine if this sum was interesting ;)

Julian Poon - 5 years, 3 months ago

Is there a sign error in your equation 2 ζ ( s ) ζ ( s ) = . . . 2\zeta(s)\zeta'(s)=... ? It seems to me that the LHS is negative while the RHS is positive.

Otto Bretscher - 5 years, 3 months ago

Log in to reply

That appears to be the case, did i do something wrong.

Aareyan Manzoor - 5 years, 3 months ago

Log in to reply

Just a sign error... the answer is the negative of what you found.

Otto Bretscher - 5 years, 3 months ago

Log in to reply

@Otto Bretscher right, I found my mistake, thanks for pointing it out.

Aareyan Manzoor - 5 years, 3 months ago

Log in to reply

@Aareyan Manzoor It's funny that two other people got it "right" anyways ;)

Otto Bretscher - 5 years, 3 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...