add numerical recipes library

lib/nr/cpp/recipes/bessj1.cpp

@@ -0,0 +1,29 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::bessj1(const DP x)
+{
+	DP ax,z,xx,y,ans,ans1,ans2;
+
+	if ((ax=fabs(x)) < 8.0) {
+		y=x*x;
+		ans1=x*(72362614232.0+y*(-7895059235.0+y*(242396853.1
+			+y*(-2972611.439+y*(15704.48260+y*(-30.16036606))))));
+		ans2=144725228442.0+y*(2300535178.0+y*(18583304.74
+			+y*(99447.43394+y*(376.9991397+y*1.0))));
+		ans=ans1/ans2;
+	} else {
+		z=8.0/ax;
+		y=z*z;
+		xx=ax-2.356194491;
+		ans1=1.0+y*(0.183105e-2+y*(-0.3516396496e-4
+			+y*(0.2457520174e-5+y*(-0.240337019e-6))));
+		ans2=0.04687499995+y*(-0.2002690873e-3
+			+y*(0.8449199096e-5+y*(-0.88228987e-6
+			+y*0.105787412e-6)));
+		ans=sqrt(0.636619772/ax)*(cos(xx)*ans1-z*sin(xx)*ans2);
+		if (x < 0.0) ans = -ans;
+	}
+	return ans;
+}