/* @(#)gser.c	17.1.1.1 (ES0-DMD) 01/25/02 17:26:59 */
/*===========================================================================
  Copyright (C) 1995 European Southern Observatory (ESO)
 
  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., 675 Massachusetss Ave, Cambridge, 
  MA 02139, USA.
 
  Corresponding concerning ESO-MIDAS should be addressed as follows:
	Internet e-mail: midas@eso.org
	Postal address: European Southern Observatory
			Data Management Division 
			Karl-Schwarzschild-Strasse 2
			D 85748 Garching bei Muenchen 
			GERMANY
===========================================================================*/

/* @(#)gser.c	17.1.1.1  (ESO)  01/25/02  17:26:59 */

#include <math.h>

#define ITMAX 100
#define EPS 3.0e-7

void gser(gamser,a,x,gln)
double a,*gamser,*gln;
float x;
{
	int n;
	double sum,del,ap;
	double gammln();
	void nrerror();

	*gln=gammln(a);
	if (x <= 0.0) {
		if (x < 0.0) nrerror("x less than 0 in routine GSER");
		*gamser=0.0;
		return;
	} else {
		ap=a;
		del=sum=1.0/a;
		for (n=1;n<=ITMAX;n++) {
			ap += 1.0;
			del *= (double)x/ap;
			sum += del;
			if (fabs(del) < fabs(sum)*EPS) {
				*gamser=sum*exp(-x+a*log(x)-(*gln));
				return;
			}
		}
		nrerror("a too large, ITMAX too small in routine GSER");
		return;
	}
}

#undef ITMAX
#undef EPS