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/* randist/chisq.c
*
* Copyright (C) 1996, 1997, 1998, 1999, 2000 James Theiler, Brian Gough
*
* This program 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 2 of the License, or (at
* your option) any later version.
*
* This program 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.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
*/
#include <config.h>
#include <math.h>
#include <gsl/gsl_sf_gamma.h>
#include <gsl/gsl_rng.h>
#include <gsl/gsl_randist.h>
/* The chisq distribution has the form
p(x) dx = (1/(2*Gamma(nu/2))) (x/2)^(nu/2 - 1) exp(-x/2) dx
for x = 0 ... +infty */
double
gsl_ran_chisq (const gsl_rng * r, const double nu)
{
double chisq = 2 * gsl_ran_gamma (r, nu / 2, 1.0);
return chisq;
}
double
gsl_ran_chisq_pdf (const double x, const double nu)
{
if (x <= 0)
{
return 0 ;
}
else
{
double p;
double lngamma = gsl_sf_lngamma (nu / 2);
p = exp ((nu / 2 - 1) * log (x/2) - x/2 - lngamma) / 2;
return p;
}
}
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