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Context

I was trying to solve a series: $$\sum_{k=2}^{\infty}\zeta'(k)x^{k-1}$$


Using the Fourier series \begin{equation} \begin{split} \log\Gamma(x)&=\frac{\ln(2\pi)}{2}+\sum_{k=1}^\infty \frac{\cos(2k\pi x)}{2k}+\sum_{k=1}^\infty \frac{\gamma+\ln(2k\pi)}{\pi k}\sin(2k\pi x)\\ B_{2n-1}(x)&=-2(2n-1)!\sum_{k=1}^\infty \frac{\sin(2k\pi x)}{(2k\pi)^{2n-1}}\\ \end{split} \end{equation} and using the generating function of the odd Bernoulli polynomials $$\frac{t}{2}\cdot\frac{\sinh((x-\frac{1}{2})t)}{\sinh(\frac{t}{2})}=\sum_{n=1}^\infty B_{2n-1}(x)\frac{t^{2n-1}}{(2n-1)!}$$ it can be shown that: $$\sum_{k=1}^\infty\zeta'(2k)z^{2k}=\frac{\gamma+\ln(2\pi)}{2}(1-\pi x\cot(\pi z))+\pi^2z^2\csc(\pi z)\int_0^1\sin(\pi t(2z-1))\log\Gamma(t)\mathrm{d}t$$

See this answer for more details.


I wanted to do the similar calculation in the case of $\zeta'(2k+1)$, but it comes naturally to me to use a function of this type: $$\text{Cl}_{2n}(x)=\sum_{k=1}^\infty \frac{\sin(kx)}{k^{2n}}$$ To then get to look for its generating function (beyond the multiplicative constants).

Question

Does anyone have any ideas on how to approach the following series? $$f(t,x)=\sum_{n=1}^\infty \text{Cl}_{2n}(x)\frac{t^{2n}}{(2n)!}$$ Where $\text{Cl}_{\nu}(z)$ is the Clausen function.

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    $\begingroup$ I was expecting you to ask this question at some point. : ) $\tag*{}$I hope you don't mind, but I've modified your question a bit to reference my answer. The full computation and subsequent short discussion may be useful to those who want to try to answer (and also, it is polite to cite your sources). Also restructured some of the first paragraph to read more cleanly, and fixed some mistakes in terminology. I do hope you're ok with the changes; if not, of course, feel free to revert them. $\endgroup$ Commented Dec 11, 2023 at 15:19
  • $\begingroup$ @C-RAM Yup no problem, you did very well $\endgroup$ Commented Dec 11, 2023 at 15:20

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