add numerical recipes library

lib/nr/k_and_r/recipes/gser.c

@@ -0,0 +1,36 @@
+
+#include <math.h>
+#define ITMAX 100
+#define EPS 3.0e-7
+
+void gser(gamser,a,x,gln)
+float *gamser,*gln,a,x;
+{
+	float gammln();
+	void nrerror();
+	int n;
+	float sum,del,ap;
+
+	*gln=gammln(a);
+	if (x <= 0.0) {
+		if (x < 0.0) nrerror("x less than 0 in routine gser");
+		*gamser=0.0;
+		return;
+	} else {
+		ap=a;
+		del=sum=1.0/a;
+		for (n=1;n<=ITMAX;n++) {
+			++ap;
+			del *= x/ap;
+			sum += del;
+			if (fabs(del) < fabs(sum)*EPS) {
+				*gamser=sum*exp(-x+a*log(x)-(*gln));
+				return;
+			}
+		}
+		nrerror("a too large, ITMAX too small in routine gser");
+		return;
+	}
+}
+#undef ITMAX
+#undef EPS