add numerical recipes library

lib/nr/k_and_r/recipes/eulsum.c

@@ -0,0 +1,29 @@
+
+#include <math.h>
+
+void eulsum(sum,term,jterm,wksp)
+float *sum,term,wksp[];
+int jterm;
+{
+	int j;
+	static int nterm;
+	float tmp,dum;
+
+	if (jterm == 1) {
+		nterm=1;
+		*sum=0.5*(wksp[1]=term);
+	} else {
+		tmp=wksp[1];
+		wksp[1]=term;
+		for (j=1;j<=nterm-1;j++) {
+			dum=wksp[j+1];
+			wksp[j+1]=0.5*(wksp[j]+tmp);
+			tmp=dum;
+		}
+		wksp[nterm+1]=0.5*(wksp[nterm]+tmp);
+		if (fabs(wksp[nterm+1]) <= fabs(wksp[nterm]))
+			*sum += (0.5*wksp[++nterm]);
+		else
+			*sum += wksp[nterm+1];
+	}
+}