Just a bit of Number Theory

S = n = 1 2 ω ( n ) n 4 n = 1 d ( n ) n 4 S=\frac{\displaystyle\sum_{n=1}^{\infty}\frac{2^{\omega(n)}}{n^4}}{\displaystyle \sum_{n=1}^{\infty} \frac{d(n)}{n^4}}

Where ω ( n ) \omega(n) counts the number of distinct prime factors of n n , and d ( n ) d(n) is the number of divisors of n n .

S = a b π c S=\frac{a}{b\pi^c} enter your answer as a + b + c a+b+c


The answer is 9459.

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1 solution

Mark Hennings
Jun 25, 2019

For any prime p p we have n = 0 2 ω ( p n ) p n s = 1 + n = 1 2 p n s = 1 + 2 p s 1 p s = 1 + p s 1 p s \sum_{n=0}^\infty \frac{2^{\omega(p^n)}}{p^{ns}} \; = \; 1 + \sum_{n=1}^\infty \frac{2}{p^{ns}} \; = \; 1 + \frac{2p^{-s}}{1-p^{-s}} \; = \; \frac{1+p^{-s}}{1-p^{-s}} for s > 1 s > 1 , and hence n = 1 2 ω ( n ) n s = p 1 + p s 1 p s = ζ ( s ) × ζ ( s ) ζ ( 2 s ) = ζ ( s ) 2 ζ ( 2 s ) \begin{aligned} \sum_{n=1}^\infty \frac{2^{\omega(n)}}{n^s} & = \; \prod_p \frac{1+p^{-s}}{1-p^{-s}} \; = \; \zeta(s) \times \frac{\zeta(s)}{\zeta(2s)} \; = \; \frac{\zeta(s)^2}{\zeta(2s)} \end{aligned} for s > 1 s > 1 , while n = 1 d ( n ) n s = ζ ( s ) 2 \sum_{n=1}^\infty \frac{d(n)}{n^s} \; = \; \zeta(s)^2 for s > 1 s > 1 , so that S s = 1 ζ ( 2 s ) s > 1 S_s \; = \; \frac{1}{\zeta(2s)} \hspace{2cm} s > 1 With s = 4 s=4 we obtain S 4 = ζ ( 8 ) 1 = 9450 π 8 S_4 = \zeta(8)^{-1} = \frac{9450}{\pi^8} , making the answer 9450 + 1 + 8 = 9459 9450 + 1 + 8 = \boxed{9459} .

Just thanks

Bin bin Channel - 1 year, 11 months ago

In the solution are you using the fact that omega is a multiplicative function.

Srikanth Tupurani - 1 year, 9 months ago

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Obviously...

Mark Hennings - 1 year, 9 months ago

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