Maximum Principles for Difference Operators

Let L be a second order elliptic differential operator of the form, 1 Lu = a ij D ij u, https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203744369/d9858e60-9246-4876-bb34-38e3d28ecc5f/content/eq1392.tif"/>