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