##### Synopsis

    #include <math.h>
    double atanh(double x)

##### Description

atanh returns the inverse hyperbolic tangent of x.

##### Fast math library

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

##### IEC 60559 math library behavior

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.

##### Portability

atanh conforms to ISO/IEC 9899:1999 (C99).