1 /** @file inifcns_gamma.cpp
3 * Implementation of Gamma-function, Beta-function, Polygamma-functions, and
4 * some related stuff. */
7 * GiNaC Copyright (C) 1999 Johannes Gutenberg University Mainz, Germany
9 * This program is free software; you can redistribute it and/or modify
10 * it under the terms of the GNU General Public License as published by
11 * the Free Software Foundation; either version 2 of the License, or
12 * (at your option) any later version.
14 * This program is distributed in the hope that it will be useful,
15 * but WITHOUT ANY WARRANTY; without even the implied warranty of
16 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
17 * GNU General Public License for more details.
19 * You should have received a copy of the GNU General Public License
20 * along with this program; if not, write to the Free Software
21 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
33 #include "relational.h"
37 #ifndef NO_GINAC_NAMESPACE
39 #endif // ndef NO_GINAC_NAMESPACE
45 static ex gamma_evalf(const ex & x)
49 END_TYPECHECK(gamma(x))
51 return gamma(ex_to_numeric(x));
54 /** Evaluation of gamma(x). Knows about integer arguments, half-integer
55 * arguments and that's it. Somebody ought to provide some good numerical
56 * evaluation some day...
58 * @exception std::domain_error("gamma_eval(): simple pole") */
59 static ex gamma_eval(const ex & x)
61 if (x.info(info_flags::numeric)) {
62 // trap integer arguments:
63 if (x.info(info_flags::integer)) {
64 // gamma(n+1) -> n! for postitive n
65 if (x.info(info_flags::posint)) {
66 return factorial(ex_to_numeric(x).sub(_num1()));
68 throw (std::domain_error("gamma_eval(): simple pole"));
71 // trap half integer arguments:
72 if ((x*2).info(info_flags::integer)) {
73 // trap positive x==(n+1/2)
74 // gamma(n+1/2) -> Pi^(1/2)*(1*3*..*(2*n-1))/(2^n)
75 if ((x*_ex2()).info(info_flags::posint)) {
76 numeric n = ex_to_numeric(x).sub(_num1_2());
77 numeric coefficient = doublefactorial(n.mul(_num2()).sub(_num1()));
78 coefficient = coefficient.div(pow(_num2(),n));
79 return coefficient * pow(Pi,_ex1_2());
81 // trap negative x==(-n+1/2)
82 // gamma(-n+1/2) -> Pi^(1/2)*(-2)^n/(1*3*..*(2*n-1))
83 numeric n = abs(ex_to_numeric(x).sub(_num1_2()));
84 numeric coefficient = pow(_num_2(), n);
85 coefficient = coefficient.div(doublefactorial(n.mul(_num2()).sub(_num1())));;
86 return coefficient*power(Pi,_ex1_2());
90 return gamma(x).hold();
93 static ex gamma_diff(const ex & x, unsigned diff_param)
95 GINAC_ASSERT(diff_param==0);
97 // d/dx log(gamma(x)) -> psi(x)
98 // d/dx gamma(x) -> psi(x)*gamma(x)
99 return psi(x)*gamma(x);
102 static ex gamma_series(const ex & x, const symbol & s, const ex & point, int order)
105 // Taylor series where there is no pole falls back to psi function
107 // On a pole at -m use the recurrence relation
108 // gamma(x) == gamma(x+1) / x
109 // from which follows
110 // series(gamma(x),x,-m,order) ==
111 // series(gamma(x+m+1)/(x*(x+1)...*(x+m)),x,-m,order+1);
112 ex xpoint = x.subs(s==point);
113 if (!xpoint.info(info_flags::integer) || xpoint.info(info_flags::positive))
114 throw do_taylor(); // caught by function::series()
115 // if we got here we have to care for a simple pole at -m:
116 numeric m = -ex_to_numeric(xpoint);
117 ex ser_numer = gamma(x+m+_ex1());
118 ex ser_denom = _ex1();
119 for (numeric p; p<=m; ++p)
121 return (ser_numer/ser_denom).series(s, point, order+1);
124 REGISTER_FUNCTION(gamma, gamma_eval, gamma_evalf, gamma_diff, gamma_series);
130 static ex beta_evalf(const ex & x, const ex & y)
135 END_TYPECHECK(beta(x,y))
137 return gamma(ex_to_numeric(x))*gamma(ex_to_numeric(y))
138 / gamma(ex_to_numeric(x+y));
141 static ex beta_eval(const ex & x, const ex & y)
143 if (x.info(info_flags::numeric) && y.info(info_flags::numeric)) {
144 numeric nx(ex_to_numeric(x));
145 numeric ny(ex_to_numeric(y));
146 // treat all problematic x and y that may not be passed into gamma,
147 // because they would throw there although beta(x,y) is well-defined
148 // using the formula beta(x,y) == (-1)^y * beta(1-x-y, y)
149 if (nx.is_real() && nx.is_integer() &&
150 ny.is_real() && ny.is_integer()) {
151 if (nx.is_negative()) {
153 return pow(_num_1(), ny)*beta(1-x-y, y);
155 throw (std::domain_error("beta_eval(): simple pole"));
157 if (ny.is_negative()) {
159 return pow(_num_1(), nx)*beta(1-y-x, x);
161 throw (std::domain_error("beta_eval(): simple pole"));
163 return gamma(x)*gamma(y)/gamma(x+y);
165 // no problem in numerator, but denominator has pole:
166 if ((nx+ny).is_real() &&
167 (nx+ny).is_integer() &&
168 !(nx+ny).is_positive())
171 return gamma(x)*gamma(y)/gamma(x+y);
174 return beta(x,y).hold();
177 static ex beta_diff(const ex & x, const ex & y, unsigned diff_param)
179 GINAC_ASSERT(diff_param<2);
182 // d/dx beta(x,y) -> (psi(x)-psi(x+y)) * beta(x,y)
184 retval = (psi(x)-psi(x+y))*beta(x,y);
185 // d/dy beta(x,y) -> (psi(y)-psi(x+y)) * beta(x,y)
187 retval = (psi(y)-psi(x+y))*beta(x,y);
191 static ex beta_series(const ex & x, const ex & y, const symbol & s, const ex & point, int order)
194 // Taylor series where there is no pole falls back to beta function
196 // On a pole at -m use the recurrence relation
197 // gamma(x) == gamma(x+1) / x
198 // from which follows
199 // series(gamma(x),x,-m,order) ==
200 // series(gamma(x+m+1)/(x*(x+1)...*(x+m)),x,-m,order+1);
201 ex xpoint = x.subs(s==point);
202 ex ypoint = y.subs(s==point);
203 if ((!xpoint.info(info_flags::integer) || xpoint.info(info_flags::positive)) &&
204 (!ypoint.info(info_flags::integer) || ypoint.info(info_flags::positive)))
205 throw do_taylor(); // caught by function::series()
206 // if we got here we have to care for a simple pole at -m:
207 throw (std::domain_error("beta_series(): please code me"));
208 /*numeric m = -ex_to_numeric(xpoint);
209 *ex ser_numer = gamma(x+m+_ex1());
210 *ex ser_denom = _ex1();
211 *for (numeric p; p<=m; ++p)
213 *return (ser_numer/ser_denom).series(s, point, order+1);*/
216 REGISTER_FUNCTION(beta, beta_eval, beta_evalf, beta_diff, beta_series);
219 // Psi-function (aka digamma-function)
222 static ex psi1_evalf(const ex & x)
226 END_TYPECHECK(psi(x))
228 return psi(ex_to_numeric(x));
231 /** Evaluation of digamma-function psi(x).
232 * Somebody ought to provide some good numerical evaluation some day... */
233 static ex psi1_eval(const ex & x)
235 if (x.info(info_flags::numeric)) {
236 if (x.info(info_flags::integer) && !x.info(info_flags::positive))
237 throw (std::domain_error("psi_eval(): simple pole"));
238 if (x.info(info_flags::positive)) {
239 // psi(n) -> 1 + 1/2 +...+ 1/(n-1) - EulerGamma
240 if (x.info(info_flags::integer)) {
242 for (numeric i(ex_to_numeric(x)-_num1()); i.is_positive(); --i)
244 return rat-EulerGamma;
246 // psi((2m+1)/2) -> 2/(2m+1) + 2/2m +...+ 2/1 - EulerGamma - 2log(2)
247 if ((_ex2()*x).info(info_flags::integer)) {
249 for (numeric i((ex_to_numeric(x)-_num1())*_num2()); i.is_positive(); i-=_num2())
250 rat += _num2()*i.inverse();
251 return rat-EulerGamma-_ex2()*log(_ex2());
253 if (x.compare(_ex1())==1) {
254 // should call numeric, since >1
258 return psi(x).hold();
261 static ex psi1_diff(const ex & x, unsigned diff_param)
263 GINAC_ASSERT(diff_param==0);
265 // d/dx psi(x) -> psi(1,x)
266 return psi(_ex1(), x);
269 static ex psi1_series(const ex & x, const symbol & s, const ex & point, int order)
272 // Taylor series where there is no pole falls back to polygamma function
274 // On a pole at -m use the recurrence relation
275 // psi(x) == psi(x+1) - 1/z
276 // from which follows
277 // series(psi(x),x,-m,order) ==
278 // series(psi(x+m+1) - 1/x - 1/(x+1) - 1/(x+m)),x,-m,order);
279 ex xpoint = x.subs(s==point);
280 if (!xpoint.info(info_flags::integer) || xpoint.info(info_flags::positive))
281 throw do_taylor(); // caught by function::series()
282 // if we got here we have to care for a simple pole at -m:
283 numeric m = -ex_to_numeric(xpoint);
285 for (numeric p; p<=m; ++p)
286 recur += power(x+p,_ex_1());
287 return (psi(x+m+_ex1())-recur).series(s, point, order);
290 const unsigned function_index_psi1 = function::register_new("psi", psi1_eval, psi1_evalf, psi1_diff, psi1_series);
293 // Psi-functions (aka polygamma-functions) psi(0,x)==psi(x)
296 static ex psi2_evalf(const ex & n, const ex & x)
301 END_TYPECHECK(psi(n,x))
303 return psi(ex_to_numeric(n), ex_to_numeric(x));
306 /** Evaluation of polygamma-function psi(n,x).
307 * Somebody ought to provide some good numerical evaluation some day... */
308 static ex psi2_eval(const ex & n, const ex & x)
310 // psi(0,x) -> psi(x)
313 // psi(-1,x) -> log(gamma(x))
314 if (n.is_equal(_ex_1()))
315 return log(gamma(x));
316 if (n.info(info_flags::numeric) && n.info(info_flags::posint) &&
317 x.info(info_flags::numeric)) {
318 numeric nn = ex_to_numeric(n);
319 numeric nx = ex_to_numeric(x);
320 if (nx.is_integer()) {
321 if (nx.is_equal(_num1()))
322 return pow(_num_1(), nn+_num1())*factorial(nn)*zeta(ex(nn+_num1()));
323 if (nx.is_positive()) {
324 // use the recurrence relation
325 // psi(n,m) == psi(n,m+1) - (-)^n * n! / m^(n+1)
326 // to relate psi(n,m) to psi(n,1):
327 // psi(n,m) == psi(n,1) + r
328 // where r == (-)^n * n! * (1^(-n-1) + ... + (m-1)^(-n-1))
330 for (numeric p(1); p<nx; ++p)
331 recur += pow(p, -nn+_num_1());
332 recur *= factorial(nn)*pow(_num_1(), nn);
333 return recur+psi(n,_ex1());
335 // for non-positive integers there is a pole:
336 throw (std::domain_error("psi2_eval(): pole"));
339 return psi(n, x).hold();
342 static ex psi2_diff(const ex & n, const ex & x, unsigned diff_param)
344 GINAC_ASSERT(diff_param<2);
348 throw(std::logic_error("cannot diff psi(n,x) with respect to n"));
350 // d/dx psi(n,x) -> psi(n+1,x)
351 return psi(n+_ex1(), x);
354 static ex psi2_series(const ex & n, const ex & x, const symbol & s, const ex & point, int order)
357 // Taylor series where there is no pole falls back to polygamma function
359 // On a pole at -m use the recurrence relation
360 // psi(n,x) == psi(n,x+1) - (-)^n * n! / x^(n+1)
361 // from which follows
362 // series(psi(x),x,-m,order) ==
363 // series(psi(x+m+1) - (-1)^n * n! * ((x)^(-n-1) + (x+1)^(-n-1) + ...
364 // ... + (x+m)^(-n-1))),x,-m,order);
365 ex xpoint = x.subs(s==point);
366 if (!xpoint.info(info_flags::integer) || xpoint.info(info_flags::positive))
367 throw do_taylor(); // caught by function::series()
368 // if we got here we have to care for a pole of order n+1 at -m:
369 numeric m = -ex_to_numeric(xpoint);
371 for (numeric p; p<=m; ++p)
372 recur += power(x+p,-n+_ex_1());
373 recur *= factorial(n)*power(_ex_1(),n);
374 return (psi(n, x+m+_ex1())-recur).series(s, point, order);
377 const unsigned function_index_psi2 = function::register_new("psi", psi2_eval, psi2_evalf, psi2_diff, psi2_series);
379 #ifndef NO_GINAC_NAMESPACE
381 #endif // ndef NO_GINAC_NAMESPACE