add numerical recipes library

lib/nr/cpp/recipes/gcf.cpp

@@ -0,0 +1,33 @@
+#include <cmath>
+#include <limits>
+#include "nr.h"
+using namespace std;
+
+void NR::gcf(DP &gammcf, const DP a, const DP x, DP &gln)
+{
+	const int ITMAX=100;
+	const DP EPS=numeric_limits<DP>::epsilon();
+	const DP FPMIN=numeric_limits<DP>::min()/EPS;
+	int i;
+	DP 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;
+}