add numerical recipes library

lib/nr/cpp/recipes/bessk1.cpp

@@ -0,0 +1,21 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::bessk1(const DP x)
+{
+	DP y,ans;
+
+	if (x <= 2.0) {
+		y=x*x/4.0;
+		ans=(log(x/2.0)*bessi1(x))+(1.0/x)*(1.0+y*(0.15443144
+			+y*(-0.67278579+y*(-0.18156897+y*(-0.1919402e-1
+			+y*(-0.110404e-2+y*(-0.4686e-4)))))));
+	} else {
+		y=2.0/x;
+		ans=(exp(-x)/sqrt(x))*(1.25331414+y*(0.23498619
+			+y*(-0.3655620e-1+y*(0.1504268e-1+y*(-0.780353e-2
+			+y*(0.325614e-2+y*(-0.68245e-3)))))));
+	}
+	return ans;
+}