add numerical recipes library

lib/nr/cpp/recipes/rtsec.cpp

@@ -0,0 +1,31 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+DP NR::rtsec(DP func(const DP), const DP x1, const DP x2, const DP xacc)
+{
+	const int MAXIT=30;
+	int j;
+	DP fl,f,dx,xl,rts;
+
+	fl=func(x1);
+	f=func(x2);
+	if (fabs(fl) < fabs(f)) {
+		rts=x1;
+		xl=x2;
+		SWAP(fl,f);
+	} else {
+		xl=x1;
+		rts=x2;
+	}
+	for (j=0;j<MAXIT;j++) {
+		dx=(xl-rts)*f/(f-fl);
+		xl=rts;
+		fl=f;
+		rts += dx;
+		f=func(rts);
+		if (fabs(dx) < xacc || f == 0.0) return rts;
+	}
+	nrerror("Maximum number of iterations exceeded in rtsec");
+	return 0.0;
+}