Suma de la serie cosx^n/n²

/polylog(2, cos(x))  for |cos(x)| <= 1
|                                     
|    oo                               
|  ____                               
|  \   `                              
|   \       n                         
<    \   cos (x)                      
|     )  -------         otherwise    
|    /       2                        
|   /       n                         
|  /___,                              
|  n = 1                              
\                                     

$$\begin{cases} \operatorname{Li}_{2}\left(\cos{\left(x \right)}\right) & \text{for}\: \left|{\cos{\left(x \right)}}\right| \leq 1 \\\sum_{n=1}^{\infty} \frac{\cos^{n}{\left(x \right)}}{n^{2}} & \text{otherwise} \end{cases}$$

Piecewise((polylog(2, cos(x)), Abs(cos(x)) <= 1), (Sum(cos(x)^n/n^2, (n, 1, oo)), True))