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The question I asked here Computing $\sum_{n=1}^{\infty} \left(\psi^{(0)}\left(\frac{1+n}{2}\right)-\psi^{(0)}\left(\frac{n}{2}\right)-\frac{1}{n}\right)$ made me think to ask for your support for another question, that is

$$\sum_{n=1}^{\infty} \frac{\displaystyle \left(\psi\left(\frac{n}{2}\right)+\frac{1}{n}\right)^2-\left(\psi\left(\frac{n+1}{2}\right)\right)^2}{n}$$

And here is a supplementary question (the alternating version)

$$\sum_{n=1}^{\infty} (-1)^{n+1}\frac{\displaystyle \left(\psi\left(\frac{n}{2}\right)+\frac{1}{n}\right)^2-\left(\psi\left(\frac{n+1}{2}\right)\right)^2}{n}$$

Before anything else I need a starting point that you might point out to me. Besides that, can
we finish all by only using series manipulation?

user 1591719
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1 Answers1

4

The first series has the value

$$\mathcal{S}:=\sum_{n=1}^{\infty}\frac{\left(\psi{\left(\frac{n}{2}\right)}+\frac{1}{n}\right)^2-\left(\psi{\left(\frac{n+1}{2}\right)}\right)^2}{n}=\frac83\ln^3{(2)}-\frac34\,\zeta{(3)}+2\gamma\,\ln^2{(2)}.$$


A useful formula here is the following addition identity for the digamma function:

$$\psi{\left(z+\frac12\right)}+\psi{\left(z\right)}=2\psi{\left(2z\right)}-2\ln{(2)}.$$

For convenience, I define the auxiliary function $\beta{\left(z\right)}$ in terms of the digamma functions as

$$\beta{\left(z\right)}:=\frac12\left[\psi{\left(\frac{z+1}{2}\right)}- \psi{\left(\frac{z}{2}\right)}\right].$$

(Note: my choice of using $\beta$ to name the function above is simply to be in accordance with the notation adopted in Gradshteyn since it is my primary reference, and it should not be confused with the Dirichlet beta function.)

The beta function can be represented as an integral via

$$\beta{\left(z\right)}=\int_{0}^{1}\frac{t^{z-1}}{1+t}\,\mathrm{d}t;~~~\small{\Re{(z)}>0}.$$


Evaluation of first sum:

$$\begin{align} \mathcal{S} &=\sum_{n=1}^{\infty}\frac{\left(\psi{\left(\frac{n}{2}\right)}+\frac{1}{n}\right)^2-\left(\psi{\left(\frac{n+1}{2}\right)}\right)^2}{n}\\ &=\sum_{n=1}^{\infty}\frac{\left[\frac{1}{n}+\psi{\left(\frac{n}{2}\right)}+\psi{\left(\frac{n+1}{2}\right)}\right]\left[\frac{1}{n}-\left(\psi{\left(\frac{n+1}{2}\right)}-\psi{\left(\frac{n}{2}\right)}\right)\right]}{n}\\ &=\sum_{n=1}^{\infty}\frac{\left[\frac{1}{n}+2\psi{\left(n\right)}-2\ln{(2)}\right]\left[\frac{1}{n}-\left(\psi{\left(\frac{n+1}{2}\right)}-\psi{\left(\frac{n}{2}\right)}\right)\right]}{n}\\ &=\sum_{n=1}^{\infty}\frac{\left[\frac{1}{n}+2\psi{\left(n\right)}\right]\left[\frac{1}{n}-\left(\psi{\left(\frac{n+1}{2}\right)}-\psi{\left(\frac{n}{2}\right)}\right)\right]}{n}\\ &~~~~~+2\ln{(2)}\sum_{n=1}^{\infty}\frac{\left[\left(\psi{\left(\frac{n+1}{2}\right)}-\psi{\left(\frac{n}{2}\right)}\right)-\frac{1}{n}\right]}{n}\\ &=\sum_{n=1}^{\infty}\frac{\left[\frac{1}{n}+2\psi{\left(n\right)}\right]\left[\frac{1}{n}-2\beta{\left(n\right)}\right]}{n}+2\ln{(2)}\sum_{n=1}^{\infty}\frac{\left[2\beta{\left(n\right)}-\frac{1}{n}\right]}{n}\\ &=\sum_{n=1}^{\infty}\left[\frac{1}{n^3}+\frac{2(\psi{\left(n\right)}-\beta{\left(n\right)})}{n^2}-\frac{4\psi{\left(n\right)}\beta{\left(n\right)}}{n}\right]+2\ln{(2)}\sum_{n=1}^{\infty}\left[\frac{2\beta{\left(n\right)}}{n}-\frac{1}{n^2}\right]\\ &=\sum_{n=1}^{\infty}\left[\frac{2(\psi{\left(n\right)}-\beta{\left(n\right)})}{n^2}-\frac{4\psi{\left(n\right)}\beta{\left(n\right)}}{n}\right]+4\ln{(2)}\sum_{n=1}^{\infty}\frac{\beta{\left(n\right)}}{n}+\zeta{(3)}-2\ln{(2)}\,\zeta{(2)}\\ &=2\sum_{n=1}^{\infty}\frac{\psi{\left(n\right)}-\beta{\left(n\right)}}{n^2}-4\sum_{n=1}^{\infty}\frac{\psi{\left(n\right)}\beta{\left(n\right)}}{n}+4\ln{(2)}\sum_{n=1}^{\infty}\frac{\beta{\left(n\right)}}{n}+\zeta{(3)}-2\ln{(2)}\,\zeta{(2)}\\ &=\zeta{(3)}-2\ln{(2)}\,\zeta{(2)}+4\ln{(2)}B_{1}+2D_{2}-2B_{2}-4\sum_{n=1}^{\infty}\frac{\psi{\left(n\right)}\beta{\left(n\right)}}{n}\\ &=\zeta{(3)}-2\ln{(2)}\,\zeta{(2)}+4\ln{(2)}B_{1}+2D_{2}-2B_{2}-4\sum_{n=1}^{\infty}\frac{\left[H_{n}-\frac{1}{n}-\gamma\right]\beta{\left(n\right)}}{n}\\ &=\zeta{(3)}-2\ln{(2)}\,\zeta{(2)}+4\ln{(2)}B_{1}+2D_{2}-2B_{2}\\ &~~~~~+4\gamma\sum_{n=1}^{\infty}\frac{\beta{\left(n\right)}}{n}+4\sum_{n=1}^{\infty}\frac{\beta{\left(n\right)}}{n^2}-4\sum_{n=1}^{\infty}\frac{H_{n}\,\beta{\left(n\right)}}{n}\\ &=\zeta{(3)}-2\ln{(2)}\,\zeta{(2)}+4\ln{(2)}B_{1}+2D_{2}-2B_{2}+4\gamma\,B_{1}+4B_{2}-4\sum_{n=1}^{\infty}\frac{H_{n}\,\beta{\left(n\right)}}{n}\\ &=\zeta{(3)}-2\ln{(2)}\,\zeta{(2)}+4\left(\ln{(2)}+\gamma\right)B_{1}+2D_{2}+2B_{2}-4\sum_{n=1}^{\infty}\frac{H_{n}\,\beta{\left(n\right)}}{n},\\ \end{align}$$

where $B_{1},B_{2},D_{2}$ have been introduced for the sake of compactness to stand for the simpler series,

$$\begin{cases} &B_{1}:=\sum_{n=1}^{\infty}\frac{\beta{\left(n\right)}}{n},\\ &B_{2}:=\sum_{n=1}^{\infty}\frac{\beta{\left(n\right)}}{n^2},\\ &D_{2}:=\sum_{n=1}^{\infty}\frac{\psi{\left(n\right)}}{n^2}.\\ \end{cases}$$

First we evaluate the series $\sum_{n=1}^{\infty}\frac{H_{n}\,\beta{\left(n\right)}}{n}$.

$$\begin{align} \sum_{n=1}^{\infty}\frac{H_{n}\,\beta{\left(n\right)}}{n} &=\sum_{n=1}^{\infty}\frac{H_{n}}{n}\int_{0}^{1}\frac{t^{n-1}}{1+t}\,\mathrm{d}t\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{1}{1+t}\sum_{n=1}^{\infty}\frac{H_{n}\,t^{n-1}}{n}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{1}{1+t}\sum_{n=1}^{\infty}\frac{t^{n-1}}{n}\int_{0}^{1}\mathrm{d}u\,\frac{1-u^{n}}{1-u}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{1}{1+t}\int_{0}^{1}\mathrm{d}u\,\sum_{n=1}^{\infty}\frac{t^{n-1}}{n}\cdot\frac{1-u^{n}}{1-u}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{1}{1+t}\int_{0}^{1}\mathrm{d}u\,\frac{1}{1-u}\sum_{n=1}^{\infty}\frac{t^{n-1}\left(1-u^{n}\right)}{n}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{1}{1+t}\int_{0}^{1}\mathrm{d}u\,\frac{1}{1-u}\left[\frac{\ln{\left(1-tu\right)}-\ln{\left(1-t\right)}}{t}\right]\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{1}{t\left(1+t\right)}\int_{0}^{1}\mathrm{d}u\,\frac{\ln{\left(\frac{1-tu}{1-t}\right)}}{1-u}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{\operatorname{Li}_{2}{\left(t\right)}+\frac12\ln^2{\left(1-t\right)}}{t\left(1+t\right)}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{\operatorname{Li}_{2}{\left(t\right)}+\frac12\ln^2{\left(1-t\right)}}{t}-\int_{0}^{1}\mathrm{d}t\,\frac{\operatorname{Li}_{2}{\left(t\right)}+\frac12\ln^2{\left(1-t\right)}}{1+t}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{\operatorname{Li}_{2}{\left(t\right)}}{t}+\int_{0}^{1}\mathrm{d}t\,\frac{\ln^2{\left(1-t\right)}}{2t}\\ &~~~~~-\int_{0}^{1}\mathrm{d}t\,\frac{\operatorname{Li}_{2}{\left(t\right)}}{1+t}-\int_{0}^{1}\mathrm{d}t\,\frac{\ln^2{\left(1-t\right)}}{2\left(1+t\right)}\\ &=2\int_{0}^{1}\mathrm{d}t\,\frac{\operatorname{Li}_{2}{\left(t\right)}}{t}-\int_{0}^{1}\mathrm{d}t\,\frac{\operatorname{Li}_{2}{\left(t\right)}}{1+t}-\int_{0}^{1}\mathrm{d}t\,\frac{\ln^2{\left(1-t\right)}}{2\left(1+t\right)},\\ \end{align}$$

where in the last line above we've made use of the following equality of integrals:

$$\begin{align} \int_{0}^{1}\mathrm{d}t\,\frac{\ln^2{\left(1-t\right)}}{2t} &=\left[\frac{\ln{\left(t\right)}\,\ln^2{\left(1-t\right)}}{2}\right]_{0}^{1}-\int_{0}^{1}\mathrm{d}t\,\frac{\ln{\left(t\right)}\,\ln{\left(1-t\right)}}{t-1}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{\ln{\left(t\right)}\,\ln{\left(1-t\right)}}{1-t}\\ &=\int_{0}^{1}\mathrm{d}u\,\frac{\ln{\left(1-u\right)}\,\ln{\left(u\right)}}{u};~~~\small{1-t=u}\\ &=\left[-\ln{\left(u\right)}\,\operatorname{Li}_{2}{\left(u\right)}\right]_{0}^{1}+\int_{0}^{1}\mathrm{d}u\,\frac{\operatorname{Li}_{2}{\left(u\right)}}{u}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{\operatorname{Li}_{2}{\left(t\right)}}{t}.\\ \end{align}$$

The first of the three integrals to be evaluated is simply $\operatorname{Li}_{3}{\left(1\right)}=\zeta{(3)}$, and the third integral turns out to be equal to $\operatorname{Li}_{3}{\left(\frac12\right)}$:

$$\begin{align} \int_{0}^{1}\mathrm{d}t\,\frac{\ln^2{\left(1-t\right)}}{2\left(1+t\right)} &=\int_{\frac12}^{0}\mathrm{d}u\,(-2)\,\frac{\ln^2{\left(2u\right)}}{2\left(2-2u\right)};~~~\small{\left[\frac{1-t}{2}=u\right]}\\ &=\int_{0}^{\frac12}\mathrm{d}u\,\frac{\ln^2{\left(2u\right)}}{2\left(1-u\right)}\\ &=\frac14\int_{0}^{1}\mathrm{d}x\,\frac{\ln^2{\left(x\right)}}{1-\left(\frac12\right)x};~~~\small{\left[2u=x\right]}\\ &=\frac14\cdot\frac{2\operatorname{Li}_{3}{\left(z\right)}}{z}\bigg{|}_{z=\frac12}\\ &=\operatorname{Li}_{3}{\left(\frac12\right)}.\\ \end{align}$$

The remaining integral may be evaluated as follows:

$$\begin{align} \int_{0}^{1}\frac{\operatorname{Li}_{2}{\left(t\right)}}{1+t}\,\mathrm{d}t &=\left[\ln{\left(1+t\right)}\,\operatorname{Li}_{2}{\left(t\right)}\right]_{0}^{1}+\int_{0}^{1}\frac{\ln{\left(1-t\right)}\,\ln{\left(t+1\right)}}{t}\,\mathrm{d}t\\ &=\ln{(2)}\,\zeta{(2)}+\int_{0}^{1}\frac{\ln{\left(1-t\right)}\,\ln{\left(t+1\right)}}{t}\,\mathrm{d}t\\ &=\ln{(2)}\,\zeta{(2)}+\int_{0}^{1}\frac{\ln^2{\left(1-t\right)}+\ln^2{\left(1+t\right)}-\ln^2{\left(\frac{1+t}{1-t}\right)}}{2t}\,\mathrm{d}t\\ &=\ln{(2)}\,\zeta{(2)}+\int_{0}^{1}\frac{\ln^2{\left(1-t\right)}}{2t}\,\mathrm{d}t+\int_{0}^{1}\frac{\ln^2{\left(1+t\right)}}{2t}\,\mathrm{d}t-\int_{0}^{1}\frac{\ln^2{\left(\frac{1+t}{1-t}\right)}}{2t}\,\mathrm{d}t\\ &=\ln{(2)}\,\zeta{(2)}+\zeta{(3)}+\frac18\zeta{(3)}-\int_{0}^{1}\frac{\ln^2{\left(\frac{1+t}{1-t}\right)}}{2t}\,\mathrm{d}t\\ &=\ln{(2)}\,\zeta{(2)}+\frac98\zeta{(3)}-\int_{0}^{1}\frac{\ln^2{\left(u\right)}}{1-u^2}\,\mathrm{d}u;~~~\small{\frac{1-t}{1+t}=u}\\ &=\ln{(2)}\,\zeta{(2)}+\frac98\zeta{(3)}-\frac74\zeta{(3)}\\ &=\ln{(2)}\,\zeta{(2)}-\frac58\zeta{(3)}.\\ \end{align}$$


$$\begin{align} B_{1} &=\sum_{n=1}^{\infty}\frac{\beta{\left(n\right)}}{n}\\ &=\sum_{n=1}^{\infty}\frac{1}{n}\int_{0}^{1}\mathrm{d}t\,\frac{t^{n-1}}{t+1}\\ &=\int_{0}^{1}\mathrm{d}t\,\sum_{n=1}^{\infty}\frac{1}{n}\left(\frac{t^{n-1}}{t+1}\right)\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{1}{t+1}\sum_{n=1}^{\infty}\frac{t^{n-1}}{n}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{1}{t\left(t+1\right)}\sum_{n=1}^{\infty}\frac{t^{n}}{n}\\ &=\int_{0}^{1}\frac{(-1)\ln{\left(1-t\right)}}{t\left(t+1\right)}\,\mathrm{d}t\\ &=-\int_{0}^{1}\frac{\ln{\left(1-t\right)}}{t}\,\mathrm{d}t+\int_{0}^{1}\frac{\ln{\left(1-t\right)}}{t+1}\,\mathrm{d}t\\ &=\zeta{(2)}+\int_{0}^{1}\frac{\ln{\left(1-t\right)}}{t+1}\,\mathrm{d}t\\ &=\zeta{(2)}+\int_{0}^{1}\frac{\ln{\left(u\right)}}{2-u}\,\mathrm{d}u;~~~\small{\left[1-t=u\right]}\\ &=\zeta{(2)}+\frac12\ln^2{(2)}-\frac12\,\zeta{(2)}\\ &=\frac12\,\zeta{(2)}+\frac12\ln^2{(2)}.\\ \end{align}$$


$$\begin{align} B_{2} &=\sum_{n=1}^{\infty}\frac{\beta{\left(n\right)}}{n^2}\\ &=\sum_{n=1}^{\infty}\frac{1}{n^2}\int_{0}^{1}\mathrm{d}t\,\frac{t^{n-1}}{t+1}\\ &=\int_{0}^{1}\mathrm{d}t\,\sum_{n=1}^{\infty}\frac{1}{n^2}\left(\frac{t^{n-1}}{t+1}\right)\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{1}{t+1}\sum_{n=1}^{\infty}\frac{t^{n-1}}{n^2}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{1}{t\left(t+1\right)}\sum_{n=1}^{\infty}\frac{t^{n}}{n^2}\\ &=\int_{0}^{1}\mathrm{d}t\,\frac{1}{t\left(t+1\right)}\,\operatorname{Li}_{2}{\left(t\right)}\\ &=\int_{0}^{1}\frac{\operatorname{Li}_{2}{\left(t\right)}}{t\left(t+1\right)}\,\mathrm{d}t\\ &=\int_{0}^{1}\frac{\operatorname{Li}_{2}{\left(t\right)}}{t}\,\mathrm{d}t-\int_{0}^{1}\frac{\operatorname{Li}_{2}{\left(t\right)}}{t+1}\,\mathrm{d}t\\ &=\zeta{(3)}-I\\ &=\frac{13}{8}\,\zeta{(3)}-\ln{(2)}\,\zeta{(2)}.\\ \end{align}$$


$D_{2}\approx0.25245=\zeta{(3)}-\gamma\,\zeta{(2)}$

$$\begin{align} D_{2} &=\sum_{n=1}^{\infty}\frac{\psi{\left(n\right)}}{n^2}\\ &=\sum_{n=1}^{\infty}\frac{1}{n^2}\left[-\gamma+\int_{0}^{1}\frac{t-t^{n}}{t-t^2}\,\mathrm{d}t\right]\\ &=-\gamma\sum_{n=1}^{\infty}\frac{1}{n^2}+\sum_{n=1}^{\infty}\frac{1}{n^2}\int_{0}^{1}\mathrm{d}t\,\frac{t-t^{n}}{t-t^2}\\ &=-\gamma\,\zeta{(2)}+\int_{0}^{1}\mathrm{d}t\,\sum_{n=1}^{\infty}\frac{1}{n^2}\left(\frac{t-t^{n}}{t-t^2}\right)\\ &=-\gamma\,\zeta{(2)}+\int_{0}^{1}\mathrm{d}t\,\frac{1}{t-t^2}\sum_{n=1}^{\infty}\frac{t-t^{n}}{n^2}\\ &=-\gamma\,\zeta{(2)}+\int_{0}^{1}\mathrm{d}t\,\frac{1}{t-t^2}\left[\zeta{(2)}\,t-\operatorname{Li}_{2}{\left(t\right)}\right]\\ &=-\gamma\,\zeta{(2)}+\int_{0}^{1}\mathrm{d}t\,\frac{\zeta{(2)}-\frac{\operatorname{Li}_{2}{\left(t\right)}}{t}}{1-t}\\ &=\zeta{(3)}-\gamma\,\zeta{(2)}.\\ \end{align}$$


Now it's just a matter of adding up all the specified terms, upon which one finds the final value stated at the top of this response.

David H
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