Function: acosh
Section: transcendental
C-Name: gacosh
Prototype: Gp
Help: acosh(x): inverse hyperbolic cosine of x.
Doc: principal branch of $\cosh^{-1}(x) = 2
  \log(\sqrt{(x+1)/2} + \sqrt{(x-1)/2})$. In particular,
 $\Re(\text{acosh}(x))\geq 0$ and
 $\Im(\text{acosh}(x))\in ]-\pi,\pi]$; if $x\in \R$ and $x<1$, then
 $\text{acosh}(x)$ is complex.
