add numerical recipes library

lib/nr/cpp/recipes/bessj0.cpp

@@ -0,0 +1,28 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::bessj0(const DP x)
+{
+	DP ax,z,xx,y,ans,ans1,ans2;
+
+	if ((ax=fabs(x)) < 8.0) {
+		y=x*x;
+		ans1=57568490574.0+y*(-13362590354.0+y*(651619640.7
+			+y*(-11214424.18+y*(77392.33017+y*(-184.9052456)))));
+		ans2=57568490411.0+y*(1029532985.0+y*(9494680.718
+			+y*(59272.64853+y*(267.8532712+y*1.0))));
+		ans=ans1/ans2;
+	} else {
+		z=8.0/ax;
+		y=z*z;
+		xx=ax-0.785398164;
+		ans1=1.0+y*(-0.1098628627e-2+y*(0.2734510407e-4
+			+y*(-0.2073370639e-5+y*0.2093887211e-6)));
+		ans2 = -0.1562499995e-1+y*(0.1430488765e-3
+			+y*(-0.6911147651e-5+y*(0.7621095161e-6
+			-y*0.934945152e-7)));
+		ans=sqrt(0.636619772/ax)*(cos(xx)*ans1-z*sin(xx)*ans2);
+	}
+	return ans;
+}