Mystical Summation that requires explaination

σ 0 ( n ) 8 1 n 2 = A \sum _{ { \sigma }_{ 0 }\left( n \right) \ge 8 }^{ }{ \frac { 1 }{ { n }^{ 2 } } } =A

Find 10000 A \displaystyle \left\lfloor 10000A \right\rfloor .

Inspiration

Bonus: Find the exact value

You may use the following approximations

π 2 9.8696044 \displaystyle { \pi }^{ 2 }\approx 9.8696044

P ( 2 ) 0.4522474 \displaystyle P\left( 2 \right) \approx 0.4522474

P ( 4 ) 0.0769931 \displaystyle P\left( 4 \right) \approx 0.0769931

P ( 6 ) 0.0170700 \displaystyle P\left( 6 \right) \approx 0.0170700

P ( 8 ) 0.0040614 \displaystyle P\left( 8 \right) \approx 0.0040614

P ( 10 ) 0.0009936 \displaystyle P\left( 10 \right) \approx 0.0009936

P ( 12 ) 0.0002460 \displaystyle P\left( 12 \right) \approx 0.0002460

Notation:

σ 0 ( n ) \displaystyle { \sigma }_{ 0 }\left( n \right) is the divisor function, it counts the number of positive factors of a number.

P ( x ) \displaystyle P\left( x \right) is the prime zeta function or P ( n ) = p p r i m e 1 p n \displaystyle P\left( n \right) =\sum _{ p\quad prime }^{ }{ \frac { 1 }{ { p }^{ n } } }

x \displaystyle \left\lfloor x \right\rfloor is the floor function.


The answer is 118.

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1 solution

Mark Hennings
Apr 19, 2016

The numbers n n with σ 0 ( n ) 7 \sigma_0(n) \le 7 are either 1 1 , or else prime powers p j p^j for 1 j 6 1 \le j \le 6 , or else either of the form p q pq or of the form p 2 q p^2q for distinct primes p p , q q . Thus ζ ( 2 ) A = σ 0 ( n ) 7 1 n 2 = 1 + j = 1 6 p 1 p 2 j + p q 1 p 2 q 2 + p q 1 p 4 q 2 = 1 + j = 1 6 P ( 2 j ) + 1 2 [ P ( 2 ) 2 P ( 4 ) ] + [ P ( 2 ) P ( 4 ) P ( 6 ) ] \begin{array}{rcl} \displaystyle \zeta(2) - A & = & \displaystyle \sum_{\sigma_0(n) \le 7} \frac{1}{n^2} \; = \; 1 + \sum_{j=1}^6 \sum_{p} \frac{1}{p^{2j}} + \sum_{p \neq q} \frac{1}{p^2q^2} + \sum_{p \neq q} \frac{1}{p^4q^2} \\ & = & \displaystyle 1 + \sum_{j=1}^6 P(2j) + \tfrac12\big[P(2)^2 - P(4)\big] + \big[P(2)P(4) - P(6)\big] \end{array} making A A equal to 0.0118052... 0.0118052... , and the answer 118 \boxed{118} .

excellently done

Joel Yip - 5 years, 1 month ago

The fancy part is that you don't even need to know what P ( 6 ) P(6) is.

Patrick Corn - 2 years, 7 months ago

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