add numerical recipes library

lib/nr/cpp/recipes/probks.cpp

@@ -0,0 +1,20 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::probks(const DP alam)
+{
+	const DP EPS1=1.0e-6,EPS2=1.0e-16;
+	int j;
+	DP a2,fac=2.0,sum=0.0,term,termbf=0.0;
+
+	a2 = -2.0*alam*alam;
+	for (j=1;j<=100;j++) {
+		term=fac*exp(a2*j*j);
+		sum += term;
+		if (fabs(term) <= EPS1*termbf || fabs(term) <= EPS2*sum) return sum;
+		fac = -fac;
+		termbf=fabs(term);
+	}
+	return 1.0;
+}