Function: atanh
Section: transcendental
C-Name: gatanh
Prototype: Gp
Help: atanh(x): inverse hyperbolic tangent of x.
Doc: principal branch of $\text{tanh}^{-1}(x) = \log ((1+x)/(1-x)) / 2$. In
 particular the imaginary part of $\text{atanh}(x)$ belongs to
 $[-\pi/2,\pi/2]$; if $x\in \R$ and $|x|>1$ then $\text{atanh}(x)$ is complex.
