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;
}