\[\frac{\,{\partial}^{4}u_{0,0}}{\,{\partial}^{2}x\,{\partial}^{2}y}=\frac{1}{h^%
{4}}\,(u_{1,1}+u_{-1,1}+u_{1,-1}+u_{-1,-1}-2u_{1,0}-2u_{-1,0}-2u_{0,1}-2u_{0,-%
1}+4u_{0,0})+O\left(h^{2}\right).\]