add numerical recipes library

lib/nr/ansi/recipes/relax.c

@@ -0,0 +1,16 @@
+
+void relax(double **u, double **rhs, int n)
+{
+	int i,ipass,isw,j,jsw=1;
+	double h,h2;
+
+	h=1.0/(n-1);
+	h2=h*h;
+	for (ipass=1;ipass<=2;ipass++,jsw=3-jsw) {
+		isw=jsw;
+		for (j=2;j<n;j++,isw=3-isw)
+			for (i=isw+1;i<n;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]);
+	}
+}