add numerical recipes library

lib/nr/cpp/recipes/lubksb.cpp

@@ -0,0 +1,24 @@
+#include "nr.h"
+
+void NR::lubksb(Mat_I_DP &a, Vec_I_INT &indx, Vec_IO_DP &b)
+{
+	int i,ii=0,ip,j;
+	DP sum;
+
+	int n=a.nrows();
+	for (i=0;i<n;i++) {
+		ip=indx[i];
+		sum=b[ip];
+		b[ip]=b[i];
+		if (ii != 0)
+			for (j=ii-1;j<i;j++) sum -= a[i][j]*b[j];
+		else if (sum != 0.0)
+			ii=i+1;
+		b[i]=sum;
+	}
+	for (i=n-1;i>=0;i--) {
+		sum=b[i];
+		for (j=i+1;j<n;j++) sum -= a[i][j]*b[j];
+		b[i]=sum/a[i][i];
+	}
+}