# Clausen cosine

Let $s \in \mathbb{C}$. The Clausen cosine function $\tilde{\mathrm{Cl}}_s \colon \mathbb{C} \rightarrow \mathbb{C}$ is defined as the analytic continuation of the series $$\tilde{\mathrm{Cl}}_s(z)=\displaystyle\sum_{k=1}^{\infty} \dfrac{\cos(kz)}{k^s},$$ where $\cos$ denotes cosine.