add numerical recipes library

lib/nr/k_and_r/recipes/resid.c

@@ -0,0 +1,17 @@
+
+void resid(res,u,rhs,n)
+double **res,**rhs,**u;
+int n;
+{
+	int i,j;
+	double h,h2i;
+
+	h=1.0/(n-1);
+	h2i=1.0/(h*h);
+	for (j=2;j<n;j++)
+		for (i=2;i<n;i++)
+			res[i][j] = -h2i*(u[i+1][j]+u[i-1][j]+u[i][j+1]+u[i][j-1]-
+					4.0*u[i][j])+rhs[i][j];
+	for (i=1;i<=n;i++)
+		res[i][1]=res[i][n]=res[1][i]=res[n][i]=0.0;
+}