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- /* randist/chisq.c
- *
- * Copyright (C) 1996, 1997, 1998, 1999, 2000, 2007 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 3 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 "gsl__config.h"
- #include <math.h>
- #include "gsl_sf_gamma.h"
- #include "gsl_rng.h"
- #include "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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