\[\operatorname{arctanh}z=\frac{z}{1-z^{2}}\*{\left(1+\frac{2}{3}\frac{z^{2}}{z^%
{2}-1}+\frac{2\cdot 4}{3\cdot 5}\left(\frac{z^{2}}{z^{2}-1}\right)^{2}+\cdots%
\right)},\]