<s>[INST] A comprehensive problem about $\sum_{n=1}^\infty {\frac{{n\cosh \left( {nx} \right)}}{{\sinh \left( {n\pi } \right)}}} $ Prolem:
${\mathop {\lim }\limits_{x \to {\pi ^ - }} \mathop {\lim }\limits_{n \to \infty } \left( {{n^2}\left( {1 - \sin \frac{\pi }{{2n}} \cdot \sum\limits_{k = 1}^{n - 1} {\sin \left( {\frac{{\left( {2\left\lfloor {\sqrt {kn} } \right\rfloor  + 1} \right)\pi }}{{2n}}} \right)} } \right)\left( {\frac{1}{{4{{\cos }^2}\frac{x}{2}}} - \sum\limits_{n = 1}^\infty  {\frac{{n\cosh \left( {nx} \right)}}{{\sinh \left( {n\pi } \right)}}}  - \frac{1}{{12}}} \right)} \right)}$, 
where $\lfloor\cdot\rfloor$ is greatest integer function.
Add: In my opinion, we must solve
$$\mathop {\lim }\limits_{n \to \infty } {n^2}\left( {1 - \sin \frac{\pi }{{2n}} \cdot \sum\limits_{k = 1}^{n - 1} {\sin \left( {\frac{{\left( {2\left\lfloor {\sqrt {kn} } \right\rfloor  + 1} \right)\pi }}{{2n}}} \right)} } \right)$$
and 
$$\mathop {\lim }\limits_{x \to {\pi ^ - }} \left( {\frac{1}{{4{{\cos }^2}\frac{x}{2}}} - \sum\limits_{n = 1}^\infty  {\frac{{n\cosh \left( {nx} \right)}}{{\sinh \left( {n\pi } \right)}}}  - \frac{1}{{12}}} \right).$$
But how can we obtain them?
And in this, we have
$$\sum\limits_{k = 1}^{n - 1} {\sin \left( {\frac{{\left( {2\left\lfloor {\sqrt {kn} } \right\rfloor  + 1} \right)\pi }}{{2n}}} \right)}  = \cot \left( {\frac{\pi }{{2n}}} \right)\cos \left( {\frac{\pi }{{2n}}} \right).$$
So we have 
\begin{align*}
&\mathop {\lim }\limits_{n \to \infty } {n^2}\left( {1 - \sin \frac{\pi }{{2n}} \cdot \sum\limits_{k = 1}^{n - 1} {\sin \left( {\frac{{\left( {2\left\lfloor {\sqrt {kn} } \right\rfloor  + 1} \right)\pi }}{{2n}}} \right)} } \right) \\
=& \mathop {\lim }\limits_{n \to \infty } {n^2}\left( {1 - \sin \frac{\pi }{{2n}} \cdot \cot \left( {\frac{\pi }{{2n}}} \right)\cos \left( {\frac{\pi }{{2n}}} \right)} \right)\\
 = &\mathop {\lim }\limits_{n \to \infty } {n^2}\left( {1 - {{\cos }^2}\left( {\frac{\pi }{{2n}}} \right)} \right) = \mathop {\lim }\limits_{n \to \infty } {n^2}{\sin ^2}\left( {\frac{\pi }{{2n}}} \right) = \frac{{{\pi ^2}}}{4}
\end{align*}
What about the next one? How to solve it?
 [/INST] Let $$f(x)=\sum_{n=1}^{\infty}\dfrac{\sinh{(nx)}}{\sinh{(n\pi)}}$$
so
$$f'(x)=\sum_{n=1}^{+\infty}\dfrac{n\cosh{(nx)}}{\sinh{(n\pi)}}$$
and other hand
$$f(x)=\dfrac{1}{e^{\pi-x}-1}+\sum_{n=1}^{\infty}\dfrac{e^{-(\pi-x)n}(e^{-2n\pi}-e^{-2n\pi})}{1-e^{-2n\pi}}$$
so
$$f'(x)=\dfrac{e^{\pi-x}}{(e^{\pi-x}-1)^2}+\sum_{n=1}^{\infty}\dfrac{ne^{-(\pi-x)n}(e^{-2n\pi}-e^{2nx})+2ne^{-(\pi-x)n}e^{-2nx}}{1-e^{-2n\pi}}$$
and it not easy to prove
$$\lim_{x\to \pi^{-}}\sum_{n=1}^{\infty}\dfrac{ne^{-(\pi-x)n}(e^{-2n\pi}-e^{2nx})+2ne^{-(\pi-x)n}e^{-2nx}}{1-e^{-2n\pi}}=2\sum_{n=1}^{\infty}\dfrac{n}{e^{2n\pi}-1}=\dfrac{1}{12}-\dfrac{1}{4\pi}$$
so you can prove by done
</s>