add numerical recipes library

lib/nr/cpp/recipes/gser.cpp

@@ -0,0 +1,33 @@
+#include <cmath>
+#include <limits>
+#include "nr.h"
+using namespace std;
+
+void NR::gser(DP &gamser, const DP a, const DP x, DP &gln)
+{
+	const int ITMAX=100;
+	const DP EPS=numeric_limits<DP>::epsilon();
+	int n;
+	DP sum,del,ap;
+
+	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=0;n<ITMAX;n++) {
+			++ap;
+			del *= 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;
+	}
+}