void resid(double **res, double **u, double **rhs, 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;}