\[\frac{\,{\partial}^{2}u_{0,0}}{\,\partial x\,\partial y}=-\frac{1}{2h^{2}}\,(u%
_{1,0}+u_{-1,0}+u_{0,1}+u_{0,-1}-2u_{0,0}-u_{1,1}-u_{-1,-1})+O\left(h^{2}%
\right).\]