#include <math.h>
double atanh(double x)
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 with signal.
If x is NaN, atanh returns that NaN with no signal.
If x is 1, atanh returns ∞ with signal.
If x is -1, atanh returns -∞ with signal.
atanh conforms to ISO/IEC 9899:1999 (C99).