How can one evaluate the sum $\displaystyle\sum_{n=1}^{\infty}\frac{\left(H_{2n}-\frac{1}{2}H_{n}\right)^{2}}{n^{2}}$? Here $H_{n}$ denotes the $n$-th harmonic number.

closed as off-topic by Masacroso, Clarinetist, T. Bongers, user10354138, KReiser Dec 7 at 8:20

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "This question is missing context or other details: Please provide additional context, which ideally explains why the question is relevant to you and our community. Some forms of context include: background and motivation, relevant definitions, source, possible strategies, your current progress, why the question is interesting or important, etc." – Masacroso, Clarinetist, T. Bongers, user10354138, KReiser
If this question can be reworded to fit the rules in the help center, please edit the question.

  • Do you have a conjecture for the value? Where did this show up? Mathematica is taking a particularly long time evaluating this ... – Sandeep Silwal Dec 6 at 22:38
  • 1
    Unfortunately, I have no conjecture. I'm curious of the value of this sum. The similar sum $\sum_{n=1}^{\infty}\frac{H_{n}^{2}}{n^{2}}$ is known to be $\frac{17}{4}\zeta(4).$ – Isak Dec 6 at 22:50
  • The answer is $$\frac{\pi^4}{32}$$ – user178256 Dec 6 at 22:54
  • 1
    How can this be proven? – Isak Dec 6 at 23:19

There's a nasty trick here:

$$ \frac{H_{2n}-\frac{1}{2}H_n}{n}=\frac{1}{n}\sum_{k=1}^{n}\frac{1}{2k-1}=\int_{0}^{1}x^{2n}\cdot \log\left(\frac{1+x}{1-x}\right)\frac{dx}{x}.\tag{1} $$ In particular $$ \sum_{n\geq 1}\left(\frac{H_{2n}-\frac{1}{2}H_n}{n}\right)^2=\iint_{(0,1)^2}\frac{x^2 y^2}{1-x^2 y^2}\log\left(\frac{1+x}{1-x}\right)\log\left(\frac{1+y}{1-y}\right)\frac{dx\,dy}{xy}.\tag{2} $$ Since $x\mapsto\frac{1-x}{1+x}$ is an involution the last integral equals $$ \iint_{(0,1)^2}\frac{(1-x)(1-y)\log(x)\log(y)}{(1+x)(1+y)(x+y)(1+xy)}\,dx\,dy \tag{3}$$ which is fairly simple to compute via Maclaurin series. It is, indeed, $\color{red}{\frac{\pi^4}{32}}$.

  • nasty, may be, but beautiful for sure ! – Claude Leibovici Dec 7 at 5:45
  • Thank You very much! – Isak Dec 7 at 10:22

Not the answer you're looking for? Browse other questions tagged or ask your own question.