Riemann everywhere

Calculus Level 5

n = 1 ψ 1 ( n ) n 2 = A B ζ ( C ) \sum _{ n=1 }^{ \infty }{ \dfrac { { \psi }^{ 1 }\left( n \right) }{ n^2 } } =\frac{A}{B} \zeta \left( C\right) If the equation above is true for some positive integers A A , B B , and C C , with A , B A,B coprime, find A + B + C A+B+C .



The answer is 15.

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1 solution

Starting with, S = n 1 ψ ( 1 ) ( n ) n 2 = n 1 ζ ( 2 ) H n 1 ( 2 ) n 2 = ζ 2 ( 2 ) n 1 H n ( 2 ) 1 n 2 n 2 = ζ 2 ( 2 ) + ζ ( 4 ) n 1 H n ( 2 ) n 2 \displaystyle S=\sum_{n\ge 1} \frac{\psi^{(1)}(n)}{n^2} = \sum_{n\ge 1} \frac{\zeta(2)-H_{n-1}^{(2)}}{n^2} = \zeta^2 (2) - \sum_{n\ge 1}\frac{H_{n}^{(2)}-\frac{1}{n^2}}{n^2} = \zeta^2 (2) + \zeta(4) - \sum_{n\ge 1} \frac{H_{n}^{(2)}}{n^2}

Using the generalised harmonic summation , n = 1 H n ( m ) n m = 1 2 ( ζ 2 ( m ) + ζ ( 2 m ) ) \displaystyle \sum_{n=1}^{\infty} \frac{H_{n}^{(m)}}{n^m} = \frac{1}{2}( \zeta^2(m) +\zeta(2m)) for m 2 m\ge 2

Using above we have, S = 1 2 ( ζ 2 ( 2 ) + ζ ( 4 ) ) = 7 π 4 360 = 7 4 ζ ( 4 ) \displaystyle S= \frac{1}{2} (\zeta^2(2) + \zeta(4)) = \frac{7\pi^4}{360}=\frac{7}{4}\zeta(4)

Hence the answer : 7 + 4 + 4 = 15 \boxed{7+4+4=15}

Very Fast and great work.

Refaat M. Sayed - 4 years, 10 months ago

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Yes ! Nice problem

Aditya Narayan Sharma - 4 years, 10 months ago

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