add numerical recipes library

lib/nr/k_and_r/recipes/bessi.c

@@ -0,0 +1,40 @@
+
+#include <math.h>
+#define ACC 40.0
+#define BIGNO 1.0e10
+#define BIGNI 1.0e-10
+
+float bessi(n,x)
+float x;
+int n;
+{
+	float bessi0();
+	void nrerror();
+	int j;
+	float bi,bim,bip,tox,ans;
+
+	if (n < 2) nrerror("Index n less than 2 in bessi");
+	if (x == 0.0)
+		return 0.0;
+	else {
+		tox=2.0/fabs(x);
+		bip=ans=0.0;
+		bi=1.0;
+		for (j=2*(n+(int) sqrt(ACC*n));j>0;j--) {
+			bim=bip+j*tox*bi;
+			bip=bi;
+			bi=bim;
+			if (fabs(bi) > BIGNO) {
+				ans *= BIGNI;
+				bi *= BIGNI;
+				bip *= BIGNI;
+			}
+			if (j == n) ans=bip;
+		}
+		ans *= bessi0(x)/bi;
+		return x < 0.0 && (n & 1) ? -ans : ans;
+	}
+}
+#undef ACC
+#undef BIGNO
+#undef BIGNI