add numerical recipes library

lib/nr/cpp/recipes/eulsum.cpp

@@ -0,0 +1,29 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+void NR::eulsum(DP &sum, const DP term, const int jterm, Vec_IO_DP &wksp)
+{
+	int j;
+	static int nterm;
+	DP tmp,dum;
+
+	if (jterm == 0) {
+		nterm=1;
+		sum=0.5*(wksp[0]=term);
+	} else {
+		if (nterm+1 > wksp.size()) nrerror("wksp too small in euler");
+		tmp=wksp[0];
+		wksp[0]=term;
+		for (j=1;j<nterm;j++) {
+			dum=wksp[j];
+			wksp[j]=0.5*(wksp[j-1]+tmp);
+			tmp=dum;
+		}
+		wksp[nterm]=0.5*(wksp[nterm-1]+tmp);
+		if (fabs(wksp[nterm]) <= fabs(wksp[nterm-1]))
+			sum += (0.5*wksp[nterm++]);
+		else
+			sum += wksp[nterm];
+	}
+}