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- /* randist/nbinomial.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_rng.h"
- #include "gsl_randist.h"
- #include "gsl_sf_gamma.h"
- /* The negative binomial distribution has the form,
- prob(k) = Gamma(n + k)/(Gamma(n) Gamma(k + 1)) p^n (1-p)^k
- for k = 0, 1, ... . Note that n does not have to be an integer.
- This is the Leger's algorithm (given in the answers in Knuth) */
- unsigned int
- gsl_ran_negative_binomial (const gsl_rng * r, double p, double n)
- {
- double X = gsl_ran_gamma (r, n, 1.0) ;
- unsigned int k = gsl_ran_poisson (r, X*(1-p)/p) ;
- return k ;
- }
- double
- gsl_ran_negative_binomial_pdf (const unsigned int k, const double p, double n)
- {
- double P;
- double f = gsl_sf_lngamma (k + n) ;
- double a = gsl_sf_lngamma (n) ;
- double b = gsl_sf_lngamma (k + 1.0) ;
- P = exp(f-a-b) * pow (p, n) * pow (1 - p, (double)k);
-
- return P;
- }
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