arccos(z) is equal to
− i × ln(z + √(z² − 1)) since
cos(φ) = cosh(i × φ).
The arccosine is
real for -1 ≤ x ≤ 1 only. Values are then between 0 and
π. (I had originally written
defined instead of real, thanks to
IWhoSawTheFace for pointing out this ridiculous error to me)