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Name	Size	Mode	Actions
array	6964	0644	edit dl rm
bessel_function.tcc	22473	0644	edit dl rm
beta_function.tcc	5995	0644	edit dl rm
ccomplex	1255	0644	edit dl rm
cctype	1412	0644	edit dl rm
cfenv	2004	0644	edit dl rm
cfloat	1380	0644	edit dl rm
cinttypes	2256	0644	edit dl rm
climits	1454	0644	edit dl rm
cmath	43806	0644	edit dl rm
complex	12384	0644	edit dl rm
complex.h	1261	0644	edit dl rm
cstdarg	1246	0644	edit dl rm
cstdbool	1344	0644	edit dl rm
cstdint	2623	0644	edit dl rm
cstdio	1482	0644	edit dl rm
cstdlib	1796	0644	edit dl rm
ctgmath	1248	0644	edit dl rm
ctime	1234	0644	edit dl rm
ctype.h	1209	0644	edit dl rm
cwchar	1718	0644	edit dl rm
cwctype	1459	0644	edit dl rm
ell_integral.tcc	27724	0644	edit dl rm
exp_integral.tcc	16009	0644	edit dl rm
fenv.h	1204	0644	edit dl rm
float.h	1209	0644	edit dl rm
functional	70545	0644	edit dl rm
functional_hash.h	6043	0644	edit dl rm
gamma.tcc	14682	0644	edit dl rm
hashtable.h	41537	0644	edit dl rm
hashtable_policy.h	25086	0644	edit dl rm
hypergeometric.tcc	28066	0644	edit dl rm
inttypes.h	1267	0644	edit dl rm
legendre_function.tcc	10909	0644	edit dl rm
limits.h	1214	0644	edit dl rm
math.h	4553	0644	edit dl rm
memory	1791	0644	edit dl rm
modified_bessel_func.tcc	16320	0644	edit dl rm
poly_hermite.tcc	3925	0644	edit dl rm
poly_laguerre.tcc	11676	0644	edit dl rm
random	1589	0644	edit dl rm
random.h	73123	0644	edit dl rm
random.tcc	53927	0644	edit dl rm
regex	92880	0644	edit dl rm
riemann_zeta.tcc	14063	0644	edit dl rm
shared_ptr.h	32608	0644	edit dl rm
special_function_util.h	5055	0644	edit dl rm
stdarg.h	1214	0644	edit dl rm
stdbool.h	1219	0644	edit dl rm
stdint.h	1214	0644	edit dl rm
stdio.h	1209	0644	edit dl rm
stdlib.h	1487	0644	edit dl rm
tgmath.h	1255	0644	edit dl rm
tuple	12119	0644	edit dl rm
type_traits	19019	0644	edit dl rm
unordered_map	1574	0644	edit dl rm
unordered_map.h	10216	0644	edit dl rm
unordered_set	1574	0644	edit dl rm
unordered_set.h	9540	0644	edit dl rm
utility	3225	0644	edit dl rm
wchar.h	1249	0644	edit dl rm
wctype.h	1255	0644	edit dl rm

Edit: /usr/include/c++/8/tr1/exp_integral.tcc (16009B)

// Special functions -*- C++ -*-

// Copyright (C) 2006-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file tr1/exp_integral.tcc
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{tr1/cmath}
 */

//
// ISO C++ 14882 TR1: 5.2  Special functions
//

//  Written by Edward Smith-Rowland based on:
//
//   (1) Handbook of Mathematical Functions,
//       Ed. by Milton Abramowitz and Irene A. Stegun,
//       Dover Publications, New-York, Section 5, pp. 228-251.
//   (2) The Gnu Scientific Library, http://www.gnu.org/software/gsl
//   (3) Numerical Recipes in C, by W. H. Press, S. A. Teukolsky,
//       W. T. Vetterling, B. P. Flannery, Cambridge University Press (1992),
//       2nd ed, pp. 222-225.
//

#ifndef _GLIBCXX_TR1_EXP_INTEGRAL_TCC
#define _GLIBCXX_TR1_EXP_INTEGRAL_TCC 1

#include "special_function_util.h"

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

#if _GLIBCXX_USE_STD_SPEC_FUNCS
#elif defined(_GLIBCXX_TR1_CMATH)
namespace tr1
{
#else
# error do not include this header directly, use <cmath> or <tr1/cmath>
#endif
  // [5.2] Special functions

// Implementation-space details.
  namespace __detail
  {
    template<typename _Tp> _Tp __expint_E1(_Tp);

/**
     *   @brief Return the exponential integral @f$ E_1(x) @f$
     *          by series summation.  This should be good
     *          for @f$ x < 1 @f$.
     * 
     *   The exponential integral is given by
     *          \f[
     *            E_1(x) = \int_{1}^{\infty} \frac{e^{-xt}}{t} dt
     *          \f]
     * 
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint_E1_series(_Tp __x)
    {
      const _Tp __eps = std::numeric_limits<_Tp>::epsilon();
      _Tp __term = _Tp(1);
      _Tp __esum = _Tp(0);
      _Tp __osum = _Tp(0);
      const unsigned int __max_iter = 1000;
      for (unsigned int __i = 1; __i < __max_iter; ++__i)
        {
          __term *= - __x / __i;
          if (std::abs(__term) < __eps)
            break;
          if (__term >= _Tp(0))
            __esum += __term / __i;
          else
            __osum += __term / __i;
        }

return - __esum - __osum
             - __numeric_constants<_Tp>::__gamma_e() - std::log(__x);
    }

/**
     *   @brief Return the exponential integral @f$ E_1(x) @f$
     *          by asymptotic expansion.
     * 
     *   The exponential integral is given by
     *          \f[
     *            E_1(x) = \int_{1}^\infty \frac{e^{-xt}}{t} dt
     *          \f]
     * 
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint_E1_asymp(_Tp __x)
    {
      _Tp __term = _Tp(1);
      _Tp __esum = _Tp(1);
      _Tp __osum = _Tp(0);
      const unsigned int __max_iter = 1000;
      for (unsigned int __i = 1; __i < __max_iter; ++__i)
        {
          _Tp __prev = __term;
          __term *= - __i / __x;
          if (std::abs(__term) > std::abs(__prev))
            break;
          if (__term >= _Tp(0))
            __esum += __term;
          else
            __osum += __term;
        }

return std::exp(- __x) * (__esum + __osum) / __x;
    }

/**
     *   @brief Return the exponential integral @f$ E_n(x) @f$
     *          by series summation.
     * 
     *   The exponential integral is given by
     *          \f[
     *            E_n(x) = \int_{1}^\infty \frac{e^{-xt}}{t^n} dt
     *          \f]
     * 
     *   @param  __n  The order of the exponential integral function.
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint_En_series(unsigned int __n, _Tp __x)
    {
      const unsigned int __max_iter = 1000;
      const _Tp __eps = std::numeric_limits<_Tp>::epsilon();
      const int __nm1 = __n - 1;
      _Tp __ans = (__nm1 != 0
                ? _Tp(1) / __nm1 : -std::log(__x)
                                   - __numeric_constants<_Tp>::__gamma_e());
      _Tp __fact = _Tp(1);
      for (int __i = 1; __i <= __max_iter; ++__i)
        {
          __fact *= -__x / _Tp(__i);
          _Tp __del;
          if ( __i != __nm1 )
            __del = -__fact / _Tp(__i - __nm1);
          else
            {
              _Tp __psi = -__numeric_constants<_Tp>::gamma_e();
              for (int __ii = 1; __ii <= __nm1; ++__ii)
                __psi += _Tp(1) / _Tp(__ii);
              __del = __fact * (__psi - std::log(__x)); 
            }
          __ans += __del;
          if (std::abs(__del) < __eps * std::abs(__ans))
            return __ans;
        }
      std::__throw_runtime_error(__N("Series summation failed "
                                     "in __expint_En_series."));
    }

/**
     *   @brief Return the exponential integral @f$ E_n(x) @f$
     *          by continued fractions.
     * 
     *   The exponential integral is given by
     *          \f[
     *            E_n(x) = \int_{1}^\infty \frac{e^{-xt}}{t^n} dt
     *          \f]
     * 
     *   @param  __n  The order of the exponential integral function.
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint_En_cont_frac(unsigned int __n, _Tp __x)
    {
      const unsigned int __max_iter = 1000;
      const _Tp __eps = std::numeric_limits<_Tp>::epsilon();
      const _Tp __fp_min = std::numeric_limits<_Tp>::min();
      const int __nm1 = __n - 1;
      _Tp __b = __x + _Tp(__n);
      _Tp __c = _Tp(1) / __fp_min;
      _Tp __d = _Tp(1) / __b;
      _Tp __h = __d;
      for ( unsigned int __i = 1; __i <= __max_iter; ++__i )
        {
          _Tp __a = -_Tp(__i * (__nm1 + __i));
          __b += _Tp(2);
          __d = _Tp(1) / (__a * __d + __b);
          __c = __b + __a / __c;
          const _Tp __del = __c * __d;
          __h *= __del;
          if (std::abs(__del - _Tp(1)) < __eps)
            {
              const _Tp __ans = __h * std::exp(-__x);
              return __ans;
            }
        }
      std::__throw_runtime_error(__N("Continued fraction failed "
                                     "in __expint_En_cont_frac."));
    }

/**
     *   @brief Return the exponential integral @f$ E_n(x) @f$
     *          by recursion.  Use upward recursion for @f$ x < n @f$
     *          and downward recursion (Miller's algorithm) otherwise.
     * 
     *   The exponential integral is given by
     *          \f[
     *            E_n(x) = \int_{1}^\infty \frac{e^{-xt}}{t^n} dt
     *          \f]
     * 
     *   @param  __n  The order of the exponential integral function.
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint_En_recursion(unsigned int __n, _Tp __x)
    {
      _Tp __En;
      _Tp __E1 = __expint_E1(__x);
      if (__x < _Tp(__n))
        {
          //  Forward recursion is stable only for n < x.
          __En = __E1;
          for (unsigned int __j = 2; __j < __n; ++__j)
            __En = (std::exp(-__x) - __x * __En) / _Tp(__j - 1);
        }
      else
        {
          //  Backward recursion is stable only for n >= x.
          __En = _Tp(1);
          const int __N = __n + 20;  //  TODO: Check this starting number.
          _Tp __save = _Tp(0);
          for (int __j = __N; __j > 0; --__j)
            {
              __En = (std::exp(-__x) - __j * __En) / __x;
              if (__j == __n)
                __save = __En;
            }
            _Tp __norm = __En / __E1;
            __En /= __norm;
        }

return __En;
    }

/**
     *   @brief Return the exponential integral @f$ Ei(x) @f$
     *          by series summation.
     * 
     *   The exponential integral is given by
     *          \f[
     *            Ei(x) = -\int_{-x}^\infty \frac{e^t}{t} dt
     *          \f]
     * 
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint_Ei_series(_Tp __x)
    {
      _Tp __term = _Tp(1);
      _Tp __sum = _Tp(0);
      const unsigned int __max_iter = 1000;
      for (unsigned int __i = 1; __i < __max_iter; ++__i)
        {
          __term *= __x / __i;
          __sum += __term / __i;
          if (__term < std::numeric_limits<_Tp>::epsilon() * __sum)
            break;
        }

return __numeric_constants<_Tp>::__gamma_e() + __sum + std::log(__x);
    }

/**
     *   @brief Return the exponential integral @f$ Ei(x) @f$
     *          by asymptotic expansion.
     * 
     *   The exponential integral is given by
     *          \f[
     *            Ei(x) = -\int_{-x}^\infty \frac{e^t}{t} dt
     *          \f]
     * 
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint_Ei_asymp(_Tp __x)
    {
      _Tp __term = _Tp(1);
      _Tp __sum = _Tp(1);
      const unsigned int __max_iter = 1000;
      for (unsigned int __i = 1; __i < __max_iter; ++__i)
        {
          _Tp __prev = __term;
          __term *= __i / __x;
          if (__term < std::numeric_limits<_Tp>::epsilon())
            break;
          if (__term >= __prev)
            break;
          __sum += __term;
        }

return std::exp(__x) * __sum / __x;
    }

/**
     *   @brief Return the exponential integral @f$ Ei(x) @f$.
     * 
     *   The exponential integral is given by
     *          \f[
     *            Ei(x) = -\int_{-x}^\infty \frac{e^t}{t} dt
     *          \f]
     * 
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint_Ei(_Tp __x)
    {
      if (__x < _Tp(0))
        return -__expint_E1(-__x);
      else if (__x < -std::log(std::numeric_limits<_Tp>::epsilon()))
        return __expint_Ei_series(__x);
      else
        return __expint_Ei_asymp(__x);
    }

/**
     *   @brief Return the exponential integral @f$ E_1(x) @f$.
     * 
     *   The exponential integral is given by
     *          \f[
     *            E_1(x) = \int_{1}^\infty \frac{e^{-xt}}{t} dt
     *          \f]
     * 
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint_E1(_Tp __x)
    {
      if (__x < _Tp(0))
        return -__expint_Ei(-__x);
      else if (__x < _Tp(1))
        return __expint_E1_series(__x);
      else if (__x < _Tp(100))  //  TODO: Find a good asymptotic switch point.
        return __expint_En_cont_frac(1, __x);
      else
        return __expint_E1_asymp(__x);
    }

/**
     *   @brief Return the exponential integral @f$ E_n(x) @f$
     *          for large argument.
     * 
     *   The exponential integral is given by
     *          \f[
     *            E_n(x) = \int_{1}^\infty \frac{e^{-xt}}{t^n} dt
     *          \f]
     * 
     *   This is something of an extension.
     * 
     *   @param  __n  The order of the exponential integral function.
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint_asymp(unsigned int __n, _Tp __x)
    {
      _Tp __term = _Tp(1);
      _Tp __sum = _Tp(1);
      for (unsigned int __i = 1; __i <= __n; ++__i)
        {
          _Tp __prev = __term;
          __term *= -(__n - __i + 1) / __x;
          if (std::abs(__term) > std::abs(__prev))
            break;
          __sum += __term;
        }

return std::exp(-__x) * __sum / __x;
    }

/**
     *   @brief Return the exponential integral @f$ E_n(x) @f$
     *          for large order.
     * 
     *   The exponential integral is given by
     *          \f[
     *            E_n(x) = \int_{1}^\infty \frac{e^{-xt}}{t^n} dt
     *          \f]
     *        
     *   This is something of an extension.
     * 
     *   @param  __n  The order of the exponential integral function.
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint_large_n(unsigned int __n, _Tp __x)
    {
      const _Tp __xpn = __x + __n;
      const _Tp __xpn2 = __xpn * __xpn;
      _Tp __term = _Tp(1);
      _Tp __sum = _Tp(1);
      for (unsigned int __i = 1; __i <= __n; ++__i)
        {
          _Tp __prev = __term;
          __term *= (__n - 2 * (__i - 1) * __x) / __xpn2;
          if (std::abs(__term) < std::numeric_limits<_Tp>::epsilon())
            break;
          __sum += __term;
        }

return std::exp(-__x) * __sum / __xpn;
    }

/**
     *   @brief Return the exponential integral @f$ E_n(x) @f$.
     * 
     *   The exponential integral is given by
     *          \f[
     *            E_n(x) = \int_{1}^\infty \frac{e^{-xt}}{t^n} dt
     *          \f]
     *   This is something of an extension.
     * 
     *   @param  __n  The order of the exponential integral function.
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    _Tp
    __expint(unsigned int __n, _Tp __x)
    {
      //  Return NaN on NaN input.
      if (__isnan(__x))
        return std::numeric_limits<_Tp>::quiet_NaN();
      else if (__n <= 1 && __x == _Tp(0))
        return std::numeric_limits<_Tp>::infinity();
      else
        {
          _Tp __E0 = std::exp(__x) / __x;
          if (__n == 0)
            return __E0;

_Tp __E1 = __expint_E1(__x);
          if (__n == 1)
            return __E1;

if (__x == _Tp(0))
            return _Tp(1) / static_cast<_Tp>(__n - 1);

_Tp __En = __expint_En_recursion(__n, __x);

return __En;
        }
    }

/**
     *   @brief Return the exponential integral @f$ Ei(x) @f$.
     * 
     *   The exponential integral is given by
     *   \f[
     *     Ei(x) = -\int_{-x}^\infty \frac{e^t}{t} dt
     *   \f]
     * 
     *   @param  __x  The argument of the exponential integral function.
     *   @return  The exponential integral.
     */
    template<typename _Tp>
    inline _Tp
    __expint(_Tp __x)
    {
      if (__isnan(__x))
        return std::numeric_limits<_Tp>::quiet_NaN();
      else
        return __expint_Ei(__x);
    }
  } // namespace __detail
#if ! _GLIBCXX_USE_STD_SPEC_FUNCS && defined(_GLIBCXX_TR1_CMATH)
} // namespace tr1
#endif

_GLIBCXX_END_NAMESPACE_VERSION
}

#endif // _GLIBCXX_TR1_EXP_INTEGRAL_TCC

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