Synopsis
double atanh(double x);
Description

atanh returns the inverse hyperbolic tangent of x.

If |x| ≥ 1, errno is set to EDOM and atanh returns HUGE_VAL.

If |x| > 1 atanh returns NaN.
If x is NaN, atanh returns that NaN.
If x is 1, atanh returns ∞.
If x is −1, atanh returns −∞.