Compute $\sum_{k=1}^\infty\frac{(-1)^{k-1}}{k^42^k{2k \choose k}}$

binomial-coefficientscalculusharmonic-numbersintegrationsequences-and-series

We have already proved in this post that

$$\sum_{k=1}^\infty\frac{(-1)^{k-1}}{k^32^k {2k\choose k}}=\frac1{4}\zeta(3)-\frac1{6}\ln^32$$

but how about proving

$$\sum_{k=1}^\infty\frac{(-1)^{k-1}}{k^42^k{2k \choose k}}=4\operatorname{Li}_4\left(\frac12\right)-\frac72\zeta(4)+\frac{13}4\ln2\zeta(3)-\ln^22\zeta(2)+\frac5{24}\ln^42\ ?$$

I managed to prove the equality above after long tedious calculations and I would like to see different methods. I am not going to put my solution in the body here as its too long so I will post it as an answer.

Special thanks to @automaticallyGenerated for helping me spot a typo.

Best Answer

From here we have $$\arcsin^2z=\frac12\sum_{k=1}^\infty\frac{(2z)^{2k}}{k^2{2k \choose k}}$$ Set $z=i\sqrt{\frac{y}{8}}$, we get

$$-\text{arcsinh}^2\left(\sqrt{\frac{y}{8}}\right)=\frac12\sum_{k=1}^\infty\frac{(-1)^{k}y^k}{k^22^k{2k \choose k}}$$ Now multiply both sides by $\frac{2\ln y}{y}$ then integrate from $y=0$ to $1$, we get

\begin{align} \sum_{k=1}^\infty\frac{(-1)^{k-1}}{k^42^k{2k \choose k}}&=-2\int_0^1\frac{\text{arcsinh}^2\left(\sqrt{\frac{y}{8}}\right)\ln y}{y}\ dy,\quad \color{red}{\text{arcsinh}\left(\sqrt{\frac{y}{8}}\right)=x}\\ &=-4\int_0^{\frac{\ln2}{2}}x^2\ln\left(8\sinh^2x\right)\coth x\ dx\\ &=-3\ln2\int_0^{\frac{\ln2}{2}}4x^2 \coth x\ dx-8\int_0^{\frac{\ln2}{2}}x^2\ln(\sinh x)\coth x\ dx\tag{1} \end{align}

The first integral is calculated here

$$\int_0^{\frac{\ln2}{2}}4x^2 \coth x\ dx=\frac1{4}\zeta(3)-\frac1{6}\ln^32\tag{2}$$

As for the second integral, we compute it as follows

\begin{align} I&=\int_0^{\frac{\ln2}{2}}x^2\ln(\sinh x)\coth x\ dx,\quad \color{red}{x=\ln y}\\ &=\int_0^{\sqrt{2}}\ln^2y\ln\left(\frac{y^2-1}{2y}\right)\left(\frac{y^2+1}{y^2-1}\right)\frac{\ dy}{y},\quad \color{red}{y^2-1=x}\\ &=\frac18\int_0^1\ln^2(1+x)\left(\ln x-\ln2-\frac12\ln(1+x)\right)\left(\frac{2}{x}-\frac{1}{1+x}\right)\ dx\\ &=\frac14\int_0^1\frac{\ln^2(1+x)\ln x}{x}\ dx-\frac14\ln 2\int_0^1\frac{\ln^2(1+x)}{x}\ dx-\frac18\int_0^1\frac{\ln^3(1+x)}{x}\ dx\\ &\quad-\frac18\underbrace{\int_0^1\frac{\ln^2(1+x)\ln x}{1+x}\ dx}_{\Large IBP}+\frac18\underbrace{\int_0^1\frac{\ln^2(1+x)}{1+x}\ dx}_{\Large\frac13\ln^32}+\frac1{16}\underbrace{\int_0^1\frac{\ln^3(1+x)}{1+x}\ dx}_{\Large\frac14\ln^42}\\ &=\frac14\underbrace{\int_0^1\frac{\ln^2(1+x)\ln x}{x}}_{\Large I_1}-\frac14\underbrace{\ln 2\int_0^1\frac{\ln^2(1+x)}{x}}_{\Large I_2}-\frac1{12}\underbrace{\int_0^1\frac{\ln^3(1+x)}{x}}_{\Large I_3}+\frac{11}{192}\ln^42\tag{3} \end{align}


Lets start with the first one and by using : $$\ln^2(1+x)=2\sum_{n=1}^\infty\frac{H_n}{n+1}(-x)^{n+1}=2\sum_{n=1}^\infty(-1)^n\left(\frac{H_n}{n}-\frac1{n^2}\right)x^n$$

We get

\begin{align} I_1&=\int_0^1\frac{\ln^2(1+x)\ln x}{x}\ dx=2\sum_{n=1}^\infty(-1)^n\left(\frac{H_n}{n}-\frac1{n^2}\right)\int_0^1x^{n-1}\ln x\ dx\\ &=2\sum_{n=1}^\infty(-1)^n\left(\frac{H_n}{n}-\frac1{n^2}\right)\left(-\frac1{n^2}\right)\\ &=-2\sum_{n=1}^\infty(-1)^n\frac{H_n}{n^3}+2\operatorname{Li}_4(-1)\\ &=-2\left(2\operatorname{Li_4}\left(\frac12\right)-\frac{11}4\zeta(4)+\frac74\ln2\zeta(3)-\frac12\ln^22\zeta(2)+\frac{1}{12}\ln^42\right)+2\left(-\frac78\zeta(4)\right)\\ &\boxed{=-4\operatorname{Li_4}\left(\frac12\right)+\frac{15}4\zeta(4)-\frac72\ln2\zeta(3)+\ln^22\zeta(2)-\frac{1}{6}\ln^42} \end{align}

Note that $\sum_{n=1}^\infty(-1)^n\frac{H_n}{n^3}$ is calculated here.


\begin{align} I_2&=\int_0^1\frac{\ln^2(1+x)}{x}\ dx\overset{x=\frac{1-y}{y}}{=}\int_{1/2}^1\frac{\ln^2x}{x(1-x)}\ dx\\ &=\int_{1/2}^1\frac{\ln^2x}{x}\ dx+\int_{1/2}^1\frac{\ln^2x}{1-x}\ dx\\ &=\frac13\ln^32+\sum_{n=1}^\infty\int_{1/2}^1x^{n-1}\ln^2x \ dx\\ &=\frac13\ln^32+\sum_{n=1}^\infty\left(-\frac{\ln^22}{n2^n}-\frac{2\ln2}{n^22^n}-\frac{2}{n^32^n}+\frac{2}{n^3}\right)\\ &=\frac13\ln^32-\ln^32-2\ln2\operatorname{Li}_2\left(\frac12\right)-2\operatorname{Li}_3\left(\frac12\right)+2\zeta(3)\\ &\boxed{=\frac14\zeta(3)} \end{align}


Similarly

\begin{align} I_3&=\int_0^1\frac{\ln^3(1+x)}{x}\ dx\overset{x=\frac{1-y}{y}}{=}-\int_{1/2}^1\frac{\ln^3x}{x(1-x)}\ dx\\ &=-\int_{1/2}^1\frac{\ln^3x}{x}\ dx-\int_{1/2}^1\frac{\ln^3x}{1-x}\ dx\\ &=\frac14\ln^42-\sum_{n=1}^\infty\int_{1/2}^1x^{n-1}\ln^3x \ dx\\ &=\frac14\ln^42-\sum_{n=1}^\infty\left(\frac{\ln^32}{n2^n}+\frac{3\ln^22}{n^22^n}+\frac{6\ln2}{n^32^n}+\frac{6}{n^42^n}-\frac{6}{n^4}\right)\\ &=\frac14\ln^42-\ln^42-3\ln^22\operatorname{Li}_2\left(\frac12\right)-6\ln2\operatorname{Li}_3\left(\frac12\right)-6\operatorname{Li}_4\left(\frac12\right)+6\zeta(4)\\ &\boxed{=-6\operatorname{Li}_4\left(\frac12\right)+6\zeta(4)-\frac{21}{4}\ln2\zeta(3)+\frac32\ln^22\zeta(2)-\frac14\ln^42} \end{align}

Note that for $I_2$ and $I_3$, we used

$$\operatorname{Li_2}\left( \frac12\right) =\frac12\zeta(2)-\frac12\ln^22$$ $$\operatorname{Li_3}\left( \frac12\right)=\frac78\zeta(3)-\frac12\ln2\zeta(2)+\frac16\ln^32$$


Plugging the boxed results of the integrals $I_1$, $I_2$ and $I_3$ in $(3)$, we get

$$I=-\frac12\operatorname{Li}_4\left(\frac12\right)+\frac7{16}\zeta(4)-\frac12\ln2\zeta(3)+\frac18\ln^22\zeta(2)+\frac{7}{192}\ln^42\tag{4}$$

Plugging the results from $(2)$ and $(4)$ in $(1)$, we get

$$\sum_{k=1}^\infty\frac{(-1)^{k-1}}{k^42^k{2k \choose k}}=4\operatorname{Li}_4\left(\frac12\right)-\frac72\zeta(4)+\frac{13}4\ln2\zeta(3)-\ln^22\zeta(2)+\frac5{24}\ln^42$$