add numerical recipes library

lib/nr/ansi/recipes/rtsec.c

@@ -0,0 +1,34 @@
+
+#include <math.h>
+#define MAXIT 30
+
+float rtsec(float (*func)(float), float x1, float x2, float xacc)
+{
+	void nrerror(char error_text[]);
+	int j;
+	float fl,f,dx,swap,xl,rts;
+
+	fl=(*func)(x1);
+	f=(*func)(x2);
+	if (fabs(fl) < fabs(f)) {
+		rts=x1;
+		xl=x2;
+		swap=fl;
+		fl=f;
+		f=swap;
+	} else {
+		xl=x1;
+		rts=x2;
+	}
+	for (j=1;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;
+}
+#undef MAXIT