text: 
  /* @(#)e_acosh.c 1.3 95/01/18 */
  /*
   * ====================================================
   * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   *
   * Developed at SunSoft, a Sun Microsystems, Inc. business.
   * Permission to use, copy, modify, and distribute this
   * software is freely granted, provided that this notice 
   * is preserved.
   * ====================================================
   *
   */
  
  #include <sys/cdefs.h>
  __FBSDID("$FreeBSD$");
  
  /* __ieee754_acosh(x)
   * Method :
   *	Based on 
   *		acosh(x) = log [ x + sqrt(x*x-1) ]
   *	we have
   *		acosh(x) := log(x)+ln2,	if x is large; else
   *		acosh(x) := log(2x-1/(sqrt(x*x-1)+x)) if x>2; else
   *		acosh(x) := log1p(t+sqrt(2.0*t+t*t)); where t=x-1.
   *
   * Special cases:
   *	acosh(x) is NaN with signal if x<1.
   *	acosh(NaN) is NaN without signal.
   */
  
  #include <float.h>
  
  #include "math.h"
  #include "math_private.h"
  
  static const double
  one	= 1.0,
  ln2	= 6.93147180559945286227e-01;  /* 0x3FE62E42, 0xFEFA39EF */
  
  double
  __ieee754_acosh(double x)
  {
  	double t;
  	int32_t hx;
  	u_int32_t lx;
  	EXTRACT_WORDS(hx,lx,x);
  	if(hx<0x3ff00000) {		/* x < 1 */
  	    return (x-x)/(x-x);
  	} else if(hx >=0x41b00000) {	/* x > 2**28 */
  	    if(hx >=0x7ff00000) {	/* x is inf of NaN */
  	        return x+x;
  	    } else 
  		return __ieee754_log(x)+ln2;	/* acosh(huge)=log(2x) */
  	} else if(((hx-0x3ff00000)|lx)==0) {
  	    return 0.0;			/* acosh(1) = 0 */
  	} else if (hx > 0x40000000) {	/* 2**28 > x > 2 */
  	    t=x*x;
  	    return __ieee754_log(2.0*x-one/(x+sqrt(t-one)));
  	} else {			/* 1<x<2 */
  	    t = x-one;
  	    return log1p(t+sqrt(2.0*t+t*t));
  	}
  }
  
  #if LDBL_MANT_DIG == 53
  __weak_reference(acosh, acoshl);
  #endif
  