Computing $\sum_{n=2}^{\infty}\frac{1}{(n^2-1)^2}=\frac{\pi^2}{12}-\frac{11}{16}$

digamma-functionintegrationsequences-and-series

I want to compute the following infinite sum

$$\sum_{n=2}^{\infty}\frac{1}{(n^2-1)^2}=\frac{\pi^2}{12}-\frac{11}{16}$$

To this goal, my strategy is to first compute $\sum_{n=2}^{\infty}\frac{1}{n^2-a^2}$, then take the derivative of the result with respect to $a$ and let $a \rightarrow 1$.

$$
\begin{aligned}
\sum_{n=2}^{\infty}\frac{1}{n^2-a^2}&=\frac{1}{2a}\sum_{n=2}^{\infty}\left(\frac{1}{n-a}-\frac{1}{n+a}\right)\\
&=\frac{1}{2a}\sum_{n=2}^{\infty}\left(\int_0^1t^{n-a-1}dt-\int_0^1t^{n+a-1}dt\right)\\
&=\frac{1}{2a}\sum_{n=2}^{\infty}\left(\int_0^1t^{-a}t^{n-1}dt-\int_0^1t^{a}t^{n-1}dt\right)\\
&=\frac{1}{2a}\left(\int_0^1t^{-a}\left(\sum_{n=2}^{\infty}t^{n-1}\right)dt-\int_0^1t^{a}\left(\sum_{n=2}^{\infty}t^{n-1}\right)dt\right)\\
&=\frac{1}{2a}\left(\int_0^1t^{-a}\left(\sum_{n=1}^{\infty}t^{n}\right)dt-\int_0^1t^{a}\left(\sum_{n=1}^{\infty}t^{n}\right)dt\right)\\
&=\frac{1}{2a}\left(\int_0^1t^{-a}\left(\frac{t}{1-t}\right)dt-\int_0^1t^{a}\left(\frac{t}{1-t}\right)dt\right)\\
&=\frac{1}{2a}\left(\int_0^1\frac{t^{1-a}}{1-t}dt-\int_0^1\frac{t^{a+1}}{1-t}dt\right)\\
&=\frac{1}{2a}\int_0^1\frac{t^{1-a}-t^{a+1}}{1-t}dt\\
\end{aligned}
$$

Recall the result

$$\int_0^1\frac{t^{z-1}-t^{w-1}}{1-t}dt=\psi(w)-\psi(z)$$

Hence

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

As a check, letting $a \rightarrow 1$ in $(1)$ I obtained

$$
\begin{aligned}
\sum_{n=2}^{\infty}\frac{1}{n^2-1}&=\frac{1}{2}\left(\psi(3)-\psi(1) \right)\\
&=\frac{1}{2}\left(-\gamma+\frac{3}{2}+\gamma \right)\\
&=\frac{3}{4}\\
\end{aligned}
$$

Which agrees with Wolfram.

Then, I differentiated $(1)$ w.r. to $a$

$$
\begin{aligned}
2a\sum_{n=2}^{\infty}\frac{1}{(n^2-a^2)^2}&=\frac{\partial }{\partial a}\left(\frac{\psi(a+2)}{2a}-\frac{\psi(2-a)}{2a} \right) \\
&=\left(\frac{2a\psi^{\prime}(a+2)-2\psi(a+2)}{4a^2}+\frac{2a\psi^{\prime}(2-a)+2\psi(2-a)}{4a^2} \right) \\
\end{aligned}
$$

$$\sum_{n=2}^{\infty}\frac{1}{(n^2-a^2)^2}=\left(\frac{a\psi^{\prime}(a+2)-\psi(a+2)}{4a^3}+\frac{a\psi^{\prime}(2-a)+\psi(2-a)}{4a^3} \right) \tag{2} $$

Letting $a=1$ in (2)

$$
\begin{aligned}
\sum_{n=2}^{\infty}\frac{1}{(n^2-1)^2}&=\left(\frac{\psi^{\prime}(3)-\psi(3)}{4}+\frac{\psi^{\prime}(1)+\psi(1)}{4} \right)\\
&=\left(\frac{2\left(\frac{\pi^2}{6}-\color{red} {\frac{5}{4}} \right)-2\left(-\gamma+\frac{3}{2} \right)}{8}+\frac{\frac{\pi^2}{3}-2\gamma}{8} \right)\\
&=\frac{\pi^2}{12}-\frac{11}{16}
\end{aligned}
$$

Edit, as pointed out by @user, $\psi^{\prime}(3)=\frac{\pi^2}{6}-\frac{5}{4}$ instead of $\psi^{\prime}(3)=\frac{\pi^2}{6}-2$, which gives now the right answer.

Best Answer

It should be

$$ \begin{aligned} \sum_{n=2}^{\infty}\frac{1}{(n^2-1)^2}&=\left(\frac{\psi^{\prime}(3)-\psi(3)}{4}+\frac{\psi^{\prime}(1)+\psi(1)}{4} \right)\\ &=\left(\frac{2\left(\frac{\pi^2}{6}-\color{red}{\frac54} \right)-2\left(-\gamma+\frac{3}{2} \right)}{8}+\frac{\frac{\pi^2}{3}-2\gamma}{8} \right)\\ &=\frac{\pi^2}{12}-\frac{11}{16} \end{aligned} $$

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