add numerical recipes library

lib/nr/k_and_r/recipes/gcf.c

@@ -0,0 +1,37 @@
+
+#include <math.h>
+#define ITMAX 100
+#define EPS 3.0e-7
+#define FPMIN 1.0e-30
+
+void gcf(gammcf,a,x,gln)
+float *gammcf,*gln,a,x;
+{
+	float gammln();
+	void nrerror();
+	int i;
+	float an,b,c,d,del,h;
+
+	*gln=gammln(a);
+	b=x+1.0-a;
+	c=1.0/FPMIN;
+	d=1.0/b;
+	h=d;
+	for (i=1;i<=ITMAX;i++) {
+		an = -i*(i-a);
+		b += 2.0;
+		d=an*d+b;
+		if (fabs(d) < FPMIN) d=FPMIN;
+		c=b+an/c;
+		if (fabs(c) < FPMIN) c=FPMIN;
+		d=1.0/d;
+		del=d*c;
+		h *= del;
+		if (fabs(del-1.0) < EPS) break;
+	}
+	if (i > ITMAX) nrerror("a too large, ITMAX too small in gcf");
+	*gammcf=exp(-x+a*log(x)-(*gln))*h;
+}
+#undef ITMAX
+#undef EPS
+#undef FPMIN