add numerical recipes library

lib/nr/k_and_r/recipes/bessi1.c

@@ -0,0 +1,24 @@
+
+#include <math.h>
+
+float bessi1(x)
+float x;
+{
+	float ax,ans;
+	double y;
+
+	if ((ax=fabs(x)) < 3.75) {
+		y=x/3.75;
+		y*=y;
+		ans=ax*(0.5+y*(0.87890594+y*(0.51498869+y*(0.15084934
+			+y*(0.2658733e-1+y*(0.301532e-2+y*0.32411e-3))))));
+	} else {
+		y=3.75/ax;
+		ans=0.2282967e-1+y*(-0.2895312e-1+y*(0.1787654e-1
+			-y*0.420059e-2));
+		ans=0.39894228+y*(-0.3988024e-1+y*(-0.362018e-2
+			+y*(0.163801e-2+y*(-0.1031555e-1+y*ans))));
+		ans *= (exp(ax)/sqrt(ax));
+	}
+	return x < 0.0 ? -ans : ans;
+}