// / / 2\ \ \
|| 2 | 3 \-3 + 3*x /*atanh(x)| |
||x *|---- + --------------------| |
|| | 2 3 | |
|| \2*x 2*x / | 2| |
||-------------------------------- for |x | <= 1|
|| 3 |
|| |
|| oo |
x*|< ____ |
|| \ ` |
|| \ 2*n |
|| \ x |
|| ) --------- otherwise |
|| / 2 |
|| / -1 + 4*n |
|| /___, |
|| n = 1 |
\\ /
$$x \left(\begin{cases} \frac{x^{2} \left(\frac{3}{2 x^{2}} + \frac{\left(3 x^{2} - 3\right) \operatorname{atanh}{\left(x \right)}}{2 x^{3}}\right)}{3} & \text{for}\: \left|{x^{2}}\right| \leq 1 \\\sum_{n=1}^{\infty} \frac{x^{2 n}}{4 n^{2} - 1} & \text{otherwise} \end{cases}\right)$$
x*Piecewise((x^2*(3/(2*x^2) + (-3 + 3*x^2)*atanh(x)/(2*x^3))/3, |x^2| <= 1), (Sum(x^(2*n)/(-1 + 4*n^2), (n, 1, oo)), True))