add numerical recipes library

lib/nr/k_and_r/recipes/factrl.c

@@ -0,0 +1,20 @@
+
+#include <math.h>
+
+float factrl(n)
+int n;
+{
+	float gammln();
+	void nrerror();
+	static int ntop=4;
+	static float a[33]={1.0,1.0,2.0,6.0,24.0};
+	int j;
+
+	if (n < 0) nrerror("Negative factorial in routine factrl");
+	if (n > 32) return exp(gammln(n+1.0));
+	while (ntop<n) {
+		j=ntop++;
+		a[ntop]=a[j]*ntop;
+	}
+	return a[n];
+}