1 // Special functions -*- C++ -*-
3 // Copyright (C) 2006-2007
4 // Free Software Foundation, Inc.
6 // This file is part of the GNU ISO C++ Library. This library is free
7 // software; you can redistribute it and/or modify it under the
8 // terms of the GNU General Public License as published by the
9 // Free Software Foundation; either version 2, or (at your option)
12 // This library is distributed in the hope that it will be useful,
13 // but WITHOUT ANY WARRANTY; without even the implied warranty of
14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 // GNU General Public License for more details.
17 // You should have received a copy of the GNU General Public License along
18 // with this library; see the file COPYING. If not, write to the Free
19 // Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
22 // As a special exception, you may use this file as part of a free software
23 // library without restriction. Specifically, if other files instantiate
24 // templates or use macros or inline functions from this file, or you compile
25 // this file and link it with other files to produce an executable, this
26 // file does not by itself cause the resulting executable to be covered by
27 // the GNU General Public License. This exception does not however
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29 // the GNU General Public License.
31 /** @file tr1/legendre_function.tcc
32 * This is an internal header file, included by other library headers.
33 * You should not attempt to use it directly.
37 // ISO C++ 14882 TR1: 5.2 Special functions
40 // Written by Edward Smith-Rowland based on:
41 // (1) Handbook of Mathematical Functions,
42 // ed. Milton Abramowitz and Irene A. Stegun,
43 // Dover Publications,
44 // Section 8, pp. 331-341
45 // (2) The Gnu Scientific Library, http://www.gnu.org/software/gsl
46 // (3) Numerical Recipes in C, by W. H. Press, S. A. Teukolsky,
47 // W. T. Vetterling, B. P. Flannery, Cambridge University Press (1992),
48 // 2nd ed, pp. 252-254
50 #ifndef _GLIBCXX_TR1_LEGENDRE_FUNCTION_TCC
51 #define _GLIBCXX_TR1_LEGENDRE_FUNCTION_TCC 1
53 #include "special_function_util.h"
60 // [5.2] Special functions
63 * @ingroup tr1_math_spec_func
68 // Implementation-space details.
74 * @brief Return the Legendre polynomial by recursion on order
77 * The Legendre function of @f$ l @f$ and @f$ x @f$,
78 * @f$ P_l(x) @f$, is defined by:
80 * P_l(x) = \frac{1}{2^l l!}\frac{d^l}{dx^l}(x^2 - 1)^{l}
83 * @param l The order of the Legendre polynomial. @f$l >= 0@f$.
84 * @param x The argument of the Legendre polynomial. @f$|x| <= 1@f$.
86 template<typename _Tp>
88 __poly_legendre_p(const unsigned int __l, const _Tp __x)
91 if ((__x < _Tp(-1)) || (__x > _Tp(+1)))
92 std::__throw_domain_error(__N("Argument out of range"
93 " in __poly_legendre_p."));
94 else if (__isnan(__x))
95 return std::numeric_limits<_Tp>::quiet_NaN();
96 else if (__x == +_Tp(1))
98 else if (__x == -_Tp(1))
99 return (__l % 2 == 1 ? -_Tp(1) : +_Tp(1));
102 _Tp __p_lm2 = _Tp(1);
111 for (unsigned int __ll = 2; __ll <= __l; ++__ll)
113 // This arrangement is supposed to be better for roundoff
114 // protection, Arfken, 2nd Ed, Eq 12.17a.
115 __p_l = _Tp(2) * __x * __p_lm1 - __p_lm2
116 - (__x * __p_lm1 - __p_lm2) / _Tp(__ll);
127 * @brief Return the associated Legendre function by recursion
130 * The associated Legendre function is derived from the Legendre function
131 * @f$ P_l(x) @f$ by the Rodruigez formula:
133 * P_l^m(x) = (1 - x^2)^{m/2}\frac{d^m}{dx^m}P_l(x)
136 * @param l The order of the associated Legendre function.
138 * @param m The order of the associated Legendre function.
140 * @param x The argument of the associated Legendre function.
143 template<typename _Tp>
145 __assoc_legendre_p(const unsigned int __l, const unsigned int __m,
149 if (__x < _Tp(-1) || __x > _Tp(+1))
150 std::__throw_domain_error(__N("Argument out of range"
151 " in __assoc_legendre_p."));
153 std::__throw_domain_error(__N("Degree out of range"
154 " in __assoc_legendre_p."));
155 else if (__isnan(__x))
156 return std::numeric_limits<_Tp>::quiet_NaN();
158 return __poly_legendre_p(__l, __x);
164 // Two square roots seem more accurate more of the time than just one.
165 _Tp __root = std::sqrt(_Tp(1) - __x) * std::sqrt(_Tp(1) + __x);
167 for (unsigned int __i = 1; __i <= __m; ++__i)
169 __p_mm *= -__fact * __root;
176 _Tp __p_mp1m = _Tp(2 * __m + 1) * __x * __p_mm;
180 _Tp __p_lm2m = __p_mm;
181 _Tp __P_lm1m = __p_mp1m;
183 for (unsigned int __j = __m + 2; __j <= __l; ++__j)
185 __p_lm = (_Tp(2 * __j - 1) * __x * __P_lm1m
186 - _Tp(__j + __m - 1) * __p_lm2m) / _Tp(__j - __m);
197 * @brief Return the spherical associated Legendre function.
199 * The spherical associated Legendre function of @f$ l @f$, @f$ m @f$,
200 * and @f$ \theta @f$ is defined as @f$ Y_l^m(\theta,0) @f$ where
202 * Y_l^m(\theta,\phi) = (-1)^m[\frac{(2l+1)}{4\pi}
203 * \frac{(l-m)!}{(l+m)!}]
204 * P_l^m(\cos\theta) \exp^{im\phi}
206 * is the spherical harmonic function and @f$ P_l^m(x) @f$ is the
207 * associated Legendre function.
209 * This function differs from the associated Legendre function by
210 * argument (@f$x = \cos(\theta)@f$) and by a normalization factor
211 * but this factor is rather large for large @f$ l @f$ and @f$ m @f$
212 * and so this function is stable for larger differences of @f$ l @f$
215 * @param l The order of the spherical associated Legendre function.
217 * @param m The order of the spherical associated Legendre function.
219 * @param theta The radian angle argument of the spherical associated
222 template <typename _Tp>
224 __sph_legendre(const unsigned int __l, const unsigned int __m,
227 if (__isnan(__theta))
228 return std::numeric_limits<_Tp>::quiet_NaN();
230 const _Tp __x = std::cos(__theta);
234 std::__throw_domain_error(__N("Bad argument "
235 "in __sph_legendre."));
239 _Tp __P = __poly_legendre_p(__l, __x);
240 _Tp __fact = std::sqrt(_Tp(2 * __l + 1)
241 / (_Tp(4) * __numeric_constants<_Tp>::__pi()));
245 else if (__x == _Tp(1) || __x == -_Tp(1))
252 // m > 0 and |x| < 1 here
254 // Starting value for recursion.
255 // Y_m^m(x) = sqrt( (2m+1)/(4pi m) gamma(m+1/2)/gamma(m) )
256 // (-1)^m (1-x^2)^(m/2) / pi^(1/4)
257 const _Tp __sgn = ( __m % 2 == 1 ? -_Tp(1) : _Tp(1));
258 const _Tp __y_mp1m_factor = __x * std::sqrt(_Tp(2 * __m + 3));
259 #if _GLIBCXX_USE_C99_MATH_TR1
260 const _Tp __lncirc = std::tr1::log1p(-__x * __x);
262 const _Tp __lncirc = std::log(_Tp(1) - __x * __x);
264 // Gamma(m+1/2) / Gamma(m)
265 #if _GLIBCXX_USE_C99_MATH_TR1
266 const _Tp __lnpoch = std::tr1::lgamma(_Tp(__m + _Tp(0.5L)))
267 - std::tr1::lgamma(_Tp(__m));
269 const _Tp __lnpoch = __log_gamma(_Tp(__m + _Tp(0.5L)))
270 - __log_gamma(_Tp(__m));
272 const _Tp __lnpre_val =
273 -_Tp(0.25L) * __numeric_constants<_Tp>::__lnpi()
274 + _Tp(0.5L) * (__lnpoch + __m * __lncirc);
275 _Tp __sr = std::sqrt((_Tp(2) + _Tp(1) / __m)
276 / (_Tp(4) * __numeric_constants<_Tp>::__pi()));
277 _Tp __y_mm = __sgn * __sr * std::exp(__lnpre_val);
278 _Tp __y_mp1m = __y_mp1m_factor * __y_mm;
284 else if (__l == __m + 1)
292 // Compute Y_l^m, l > m+1, upward recursion on l.
293 for ( int __ll = __m + 2; __ll <= __l; ++__ll)
295 const _Tp __rat1 = _Tp(__ll - __m) / _Tp(__ll + __m);
296 const _Tp __rat2 = _Tp(__ll - __m - 1) / _Tp(__ll + __m - 1);
297 const _Tp __fact1 = std::sqrt(__rat1 * _Tp(2 * __ll + 1)
298 * _Tp(2 * __ll - 1));
299 const _Tp __fact2 = std::sqrt(__rat1 * __rat2 * _Tp(2 * __ll + 1)
300 / _Tp(2 * __ll - 3));
301 __y_lm = (__x * __y_mp1m * __fact1
302 - (__ll + __m - 1) * __y_mm * __fact2) / _Tp(__ll - __m);
312 } // namespace std::tr1::__detail
314 /* @} */ // group tr1_math_spec_func
319 #endif // _GLIBCXX_TR1_LEGENDRE_FUNCTION_TCC