\[\nabla^{2}u_{0,0}=\frac{1}{12h^{2}}\left(-60u_{0,0}+16(u_{1,0}+u_{0,1}+u_{-1,0%
}+u_{0,-1})-(u_{2,0}+u_{0,2}+u_{-2,0}+u_{0,-2})\right)+O\left(h^{4}\right).\]