add numerical recipes library

lib/nr/k_and_r/recipes/bessy.c

@@ -0,0 +1,21 @@
+
+float bessy(n,x)
+float x;
+int n;
+{
+	float bessy0(),bessy1();
+	void nrerror();
+	int j;
+	float by,bym,byp,tox;
+
+	if (n < 2) nrerror("Index n less than 2 in bessy");
+	tox=2.0/x;
+	by=bessy1(x);
+	bym=bessy0(x);
+	for (j=1;j<n;j++) {
+		byp=j*tox*by-bym;
+		bym=by;
+		by=byp;
+	}
+	return by;
+}