Copyright 2010 The Go Authors. All rights reserved. Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.

package math
The original C code, the long comment, and the constants below are from FreeBSD's /usr/src/lib/msun/src/e_atanh.c and came with this notice. The go code is a simplified version of the original C. ==================================================== Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. Developed at SunPro, a Sun Microsystems, Inc. business. Permission to use, copy, modify, and distribute this software is freely granted, provided that this notice is preserved. ==================================================== __ieee754_atanh(x) Method : 1. Reduce x to positive by atanh(-x) = -atanh(x) 2. For x>=0.5 1 2x x atanh(x) = --- * log(1 + -------) = 0.5 * log1p(2 * --------) 2 1 - x 1 - x For x<0.5 atanh(x) = 0.5*log1p(2x+2x*x/(1-x)) Special cases: atanh(x) is NaN if |x| > 1 with signal; atanh(NaN) is that NaN with no signal; atanh(+-1) is +-INF with signal.
Atanh returns the inverse hyperbolic tangent of x. Special cases are: Atanh(1) = +Inf Atanh(±0) = ±0 Atanh(-1) = -Inf Atanh(x) = NaN if x < -1 or x > 1 Atanh(NaN) = NaN
func ( float64) float64

func ( float64) float64 {
special cases
	switch {
	case  < -1 ||  > 1 || IsNaN():
		return NaN()
	case  == 1:
		return Inf(1)
	case  == -1:
		return Inf(-1)
	}
	 := false
	if  < 0 {
		 = -
		 = true
	}
	var  float64
	switch {
	case  < :
		 = 
	case  < 0.5:
		 =  + 
		 = 0.5 * Log1p(+*/(1-))
	default:
		 = 0.5 * Log1p((+)/(1-))
	}
	if  {
		 = -
	}
	return