add numerical recipes library

lib/nr/cpp/recipes/zbrac.cpp

@@ -0,0 +1,23 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+bool NR::zbrac(DP func(const DP), DP &x1, DP &x2)
+{
+	const int NTRY=50;
+	const DP FACTOR=1.6;
+	int j;
+	DP f1,f2;
+
+	if (x1 == x2) nrerror("Bad initial range in zbrac");
+	f1=func(x1);
+	f2=func(x2);
+	for (j=0;j<NTRY;j++) {
+		if (f1*f2 < 0.0) return true;
+		if (fabs(f1) < fabs(f2))
+			f1=func(x1 += FACTOR*(x1-x2));
+		else
+			f2=func(x2 += FACTOR*(x2-x1));
+	}
+	return false;
+}