5 /* atanh (x) = 0.5 * log ((1.0 + x)/(1.0 - x)) */
15 return (x > 0 ? INFINITY : -INFINITY);
22 /* Rearrange formula to avoid precision loss for small x.
24 atanh(x) = 0.5 * log ((1.0 + x)/(1.0 - x))
25 = 0.5 * log1p ((1.0 + x)/(1.0 - x) - 1.0)
26 = 0.5 * log1p ((1.0 + x - 1.0 + x) /(1.0 - x))
27 = 0.5 * log1p ((2.0 * x ) / (1.0 - x)) */
28 z = 0.5 * __fast_log1p ((z + z) / (1.0 - z));
29 return x >= 0 ? z : -z;