gcf.c

C111 112

#include <math.h>

#define ITMAX 100
#define EPS 3.0e-7

void gcf(gammcf,a,x,gln)
float a,x,*gammcf,*gln;
{
	int n;
	float gold=0.0,g,fac=1.0,b1=1.0;
	float b0=0.0,anf,ana,an,a1,a0=1.0;
	float gammln();
	void nrerror();

	*gln=gammln(a);
	a1=x;
	for (n=1;n<=ITMAX;n++) {
		an=(float) n;
		ana=an-a;
		a0=(a1+a0*ana)*fac;
		b0=(b1+b0*ana)*fac;
		anf=an*fac;
		a1=x*a0+anf*a1;
		b1=x*b0+anf*b1;
		if (a1) {
			fac=1.0/a1;
			g=b1*fac;
			if (fabs((g-gold)/g) < EPS) {
				*gammcf=exp(-x+a*log(x)-(*gln))*g;
				return;
			}
			gold=g;
		}
	}
	nrerror("a too large, ITMAX too small in routine GCF");
}

#undef ITMAX
#undef EPS