atanh

Inverse hyperbolic tangent.

SYNOPSIS:

double x, y, atanh();
y = atanh(x);

DESCRIPTION:

Returns the inverse hyperbolic tangent of an argument in the range MINLOG to MAXLOG.

If |x| < 0.5, the rational form x + x**3 P(x)/Q(x) is employed. Otherwise:
        atanh(x) = 0.5 * log((1+x)/(1-x)).

ACCURACY:
                      Relative error:
arithmetic   domain     # trials      peak         rms
    DEC       -1,1        50000       2.4e-17     6.4e-18
    IEEE      -1,1        30000       1.9e-16     5.2e-17