Function: acosh
Section: transcendental
C-Name: gach
Prototype: Gp
Help: acosh(x): inverse hyperbolic cosine of x.
Doc: principal branch of $\text{cosh}^{-1}(x)$,
 i.e.~such that $\text{Im(acosh}(x))\in [0,\pi]$. If
 $x\in \R$ and $x<1$, then $\text{acosh}(x)$ is complex.
