Weird summation

Let Ω s ( n ) \Omega_s(n) be a completely additive function

(i.e. Ω s ( a b ) = Ω s ( a ) + Ω s ( b ) \Omega_s(ab)=\Omega_s(a)+\Omega_s(b) ) satisfying Ω s ( p ) = p s \Omega_s(p)=p^s where p p is a prime. Then n = 1 Ω s ( n ) n s = n = 1 f ( n ) n s \sum_{n=1}^\infty \dfrac{\Omega_{-s}(n)}{n^s} =\sum_{n=1}^\infty \dfrac{f(n)}{n^s} Find f ( 1 0 1729 ) f(10^{1729}) Note:

f f is independent of s s , i.e. d f d s = 0 \dfrac{df}{ds}=0


The answer is 3456.

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
Mar 17, 2016

First, we know n = 1 F ( n ) n s = ζ ( s ) p = p r i m e F ( p ) p s 1 \sum_{n=1}^\infty \dfrac{F(n)}{n^s}=\zeta(s)\sum_{p=prime}\dfrac{F(p)}{p^s-1} For F being a completely additive function F. proved here .


Using this we have n = 1 ω s ( n ) n s = ζ ( s ) p = p r i m e p s p s 1 = ζ ( s ) p = p r i m e ( 1 p s 1 1 p s ) = ζ ( s ) p = p r i m e 1 p s 1 ζ ( s ) P ( s ) \sum_{n=1}^\infty \dfrac{\omega_{-s}(n)}{n^s}=\zeta(s)\sum_{p=prime}\dfrac{p^{-s}}{p^s-1}\\=\zeta(s)\sum_{p=prime} \left(\dfrac{1}{p^s-1}-\dfrac{1}{p^s}\right) =\zeta(s)\sum_{p=prime} \dfrac{1}{p^s-1} -\zeta(s)P(s) By the form we used, we deduce ζ ( s ) p = p r i m e 1 p s 1 = n = 1 Ω 0 ( n ) n s \zeta(s)\sum_{p=prime} \dfrac{1}{p^s-1}=\sum_{n=1}^\infty \dfrac{\Omega_0 (n)}{n^s} and we can work out ζ ( s ) P ( s ) = n = 1 ω ( n ) n s \zeta(s)P(s)=\sum_{n=1}^\infty \dfrac{\omega (n)}{n^s} So, f ( n ) = ( Ω 0 ω ) ( n ) f(n)=(\Omega_0-\omega)(n) The result follows.

You would also have to prove that this is the only function possible, or that f(10^1729) only has one possible value (which I don't know how to).

Maybe it is safer to reduce the scope by adding that f(n) has to be an arithmetic function.

Julian Poon - 5 years, 2 months ago

Log in to reply

n = 1 f ( n ) n s n = 1 g ( n ) n s \sum_{n=1}^\infty \dfrac{f(n)}{n^s} \neq \sum_{n=1}^\infty \dfrac{g(n)}{n^s} If f and g are two different functions independent of s. This can be proved using contradiction and convolution.

Aareyan Manzoor - 5 years, 2 months ago

Log in to reply

Oh yeah, I forgot to think about "true for all s". I'll hide back in my cave for now. bye :p

Julian Poon - 5 years, 2 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...