Psi and Riemann together?

Calculus Level 5

k = 1 ψ 1 ( k ) k = n = 1 A n B \sum _{ k=1 }^{ \infty }{ \dfrac { { \psi }^{ 1 }\left( k \right) }{ k } } =\sum _{ n=1 }^{ \infty }{ \dfrac { A }{ { n }^{ B } } }

If the equation above holds true for positive integers A A and B B , find A + B A+B .


Inspired by Ishan.


The answer is 5.

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2 solutions

Ishan Singh
Feb 18, 2016

Note that, ψ ( 1 ) ( k ) = 0 1 x k 1 ln x ( 1 x ) d x \displaystyle \psi^{(1)}(k) = - \int_{0}^{1} \dfrac{x^{k-1} \ln x }{(1-x)} \mathrm{d}x

k = 1 ψ ( 1 ) ( k ) k = k = 1 0 1 x k 1 ln x k ( 1 x ) d x \displaystyle \implies \sum_{k=1}^{\infty} \dfrac{\psi^{(1)} (k)}{k} = -\sum_{k=1}^{\infty} \int_{0}^{1} \dfrac{x^{k-1} \ln x}{k \ (1-x)} \mathrm{d}x

= 0 1 ln x ln ( 1 x ) x ( 1 x ) d x \displaystyle = \int_{0}^{1} \dfrac{\ln x \ln (1-x)}{x(1-x)} \mathrm{d}x

= lim a 0 + lim b 0 + ( a ( b B ( a , b ) ) ) \displaystyle = \lim_{a \to 0^{+}} \lim_{b \to 0^{+}} \left(\dfrac{\partial}{\partial a} \left(\dfrac{\partial}{\partial b} \operatorname{B} (a,b)\right) \right)

= 2 ζ ( 3 ) = 2 \zeta (3)

A + B = 5 \implies A+B = \boxed{5}

Can just describe how to elaborate this limits ?

Kushal Bose - 4 years ago
Shivam Sharma
Apr 14, 2017

Simply do it like this , Applying Abel's summation , Let a {n} = \frac{1}{n} , b {n} = \ psi (n)
We get , \sum {n=1}^inf \frac{(H {n})}{n^2} = 2 \times \zeta(3)

0^1▒(ln⁡x ln⁡(1-x))/x(1-x) =∫ 0^1▒〖(ln⁡(1-x) )^2/x=2ζ(3) 〗

Mehdi Nazerian - 2 months, 1 week ago

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