Solve $S=\sum_{n=1}^\infty\frac{(-1)^{n+1}}{n^6{{2n}\choose{n}}}$

binomial-coefficientsindefinite-integralsintegrationsequences-and-series

I have the following sum:
$$S=\sum_{n=1}^\infty\frac{(-1)^{n+1}}{n^6{{2n}\choose{n}}}$$
I derived the following expression involving $\operatorname{arsinh}^2$. I started by evaluating the integral by transforming it into a double integral. I then used the power series for $\ln{(1+x)}$
$$\operatorname{arsinh}^2(t)=\int_0^\frac{\pi}{2}\frac{\ln{(1+t^2\sin^2{x})}}{\sin{x}}dx=\frac{1}{2}\sum_{n=1}^\infty\frac{(-1)^{n+1}(2t)^{2n}}{n^2{2n\choose n}}$$
I wanted to repeatedly integrate each side, but the dilogarithm and trilogarithm functions are difficult to integrate with. For instance:
$$\int\frac{\operatorname{arsinh}^2(\frac{\sqrt {t}}{2})}{t}dt=C-\frac{1}{2}\sum_{n=1}^\infty\frac{(-1)^{n+1}t^n}{{n^3}{2n\choose{n}}}$$
(WA solves here)

I am asking how we can evaluate this infinite sum. There surely must be a better way than brute force integration

Best Answer

This is not a pleasant result since $$S=\sum_{n=1}^\infty\frac{(-1)^{n+1}}{n^6{{2n}\choose{n}}}=\frac{1}{2} \, _7F_6\left(1,1,1,1,1,1,1;\frac{3}{2},2,2,2,2,2;-\frac{1}{4}\right)$$ which is $$S=0.49746116570182992164335272848313158317029710406952\cdots$$

Similarly $$\frac{1}{2}\sum_{n=1}^\infty\frac{(-1)^{n+1}}{{n^3}{2n\choose{n}}}t^n=\frac{1}{4} t \,\, _4F_3\left(1,1,1,1;\frac{3}{2},2,2;-\frac{t}{4}\right)$$ which, if simplified, leads to polylogarithms.

However, if you want to compute $$I=\int\frac 1 x\sinh ^{-1}\left(\frac{\sqrt{x}}{2}\right)^2\,dx$$ $$\color{red}{x=4 \sinh^2(w)} \quad \implies \quad I=2\int w^2 \coth(w)\,dw$$ $$I=-\frac{2 }{3}w^3+2 w^2 \log \left(1-e^{2 w}\right)+2 w \text{Li}_2\left(e^{2 w}\right)-\text{Li}_3\left(e^{2 w}\right)$$ $$I=-\zeta (3)+w^2+\frac{w^4}{6}-\frac{w^6}{135}+\frac{w^8}{1890}-\frac{w^{10}}{23 625}+O\left(w^{12}\right)$$

But, using the series expansion of $\coth(w)$ and integrating termwise, $$\color{red}{I=-\zeta(3)+\sum_{n=0}^\infty \frac{2^{2 n} B_{2 n} }{(n+1) (2 n)!}w^{2( n+1)}}$$ or $$\color{red}{I=-\zeta(3)+w^2+2\sum_{n=1}^\infty(-1)^{n+1}\,\frac{ \zeta (2 n)}{(n+1)\, \pi ^{2 n}}w^{2( n+1)}}$$

As @Gary commented, the power series converges for $|w|<\pi$ (this is a serious limitation even if this corresponds to $|x| < 533$).

$$I=-\zeta (3)+w^2+\frac{w^4}{6}-\frac{w^6}{135}+\frac{w^8}{1890}-\frac{w^{10}}{23 625}+O\left(w^{12}\right)$$

Warning

From a numerical point of view, you will face problems when $n$ is "large" (because of the order of operations). For this case, use the asymptotics $$ \frac{2^{2 n} B_{2 n} }{(n+1) (2 n)!}\sim (-1)^{n+1}\, \frac{2 \,\pi ^{-2 n}}{n+1}$$

(which is in a relative error smaller than $0.0001$% as soon as $n>9$) could be useful.

So, since we face an alternating series, we know in advance how many $p$ terms have to added if we write $$I=-\zeta(3)+\sum_{n=0}^p \frac{2^{2 n} B_{2 n} }{(n+1) (2 n)!}w^{2( n+1)}+\sum_{n=p+1}^\infty \frac{2^{2 n} B_{2 n} }{(n+1) (2 n)!}w^{2( n+1)}$$ $$\frac{2 \pi ^{-2 (p+1)} }{p+2}w^{2 (p+2)} ~\leq ~\epsilon \quad \implies \quad p \sim -2-\frac{1}{2 \log \left(\frac{w}{\pi }\right)}W\left(-\frac{4 \pi ^2 }{\epsilon }\log \left(\frac{w}{\pi }\right)\right)$$

As a consequence of $|w|<\pi$, $p$ varies extremely fast since, close to the limit, $$p=2\left(\frac{ \pi ^2}{\epsilon }-1\right)+\frac{8 \pi ^3 }{\epsilon ^2}(w-\pi)+O\left((w-\pi )^2\right)$$