add numerical recipes library

lib/nr/cpp/recipes/relax.cpp

@@ -0,0 +1,18 @@
+#include "nr.h"
+
+void NR::relax(Mat_IO_DP &u, Mat_I_DP &rhs)
+{
+	int i,ipass,isw,j,jsw=1;
+	DP h,h2;
+
+	int n=u.nrows();
+	h=1.0/(n-1);
+	h2=h*h;
+	for (ipass=0;ipass<2;ipass++,jsw=3-jsw) {
+		isw=jsw;
+		for (j=1;j<n-1;j++,isw=3-isw)
+			for (i=isw;i<n-1;i+=2)
+				u[i][j]=0.25*(u[i+1][j]+u[i-1][j]+u[i][j+1]
+					+u[i][j-1]-h2*rhs[i][j]);
+	}
+}