add numerical recipes library

lib/nr/k_and_r/recipes/rtnewt.c

@@ -0,0 +1,25 @@
+
+#include <math.h>
+#define JMAX 20
+
+float rtnewt(funcd,x1,x2,xacc)
+float x1,x2,xacc;
+void (*funcd)();
+{
+	void nrerror();
+	int j;
+	float df,dx,f,rtn;
+
+	rtn=0.5*(x1+x2);
+	for (j=1;j<=JMAX;j++) {
+		(*funcd)(rtn,&f,&df);
+		dx=f/df;
+		rtn -= dx;
+		if ((x1-rtn)*(rtn-x2) < 0.0)
+			nrerror("Jumped out of brackets in rtnewt");
+		if (fabs(dx) < xacc) return rtn;
+	}
+	nrerror("Maximum number of iterations exceeded in rtnewt");
+	return 0.0;
+}
+#undef JMAX