\[\frac{{\partial}^{2}u_{0,0}}{{\partial x}^{2}}=\frac{1}{3h^{2}}\,(u_{1,1}-2u_{%
0,1}+u_{-1,1}+u_{1,0}-2u_{0,0}+u_{-1,0}+u_{1,-1}-2u_{0,-1}+u_{-1,-1})+O\left(h%
^{2}\right).\]