Complex.ACosh (gb.gsl)

Function ACosh ( ) As Complex

Returns the complex hyperbolic arccosine of the complex number.

The branch cut is on the real axis, less than 1. Note that in this case we use the negative square root in formula 4.6.21 of Abramowitz & Stegun giving acosh(z) = log(z - sqrt(z^2 - 1)).