Structure Preserving Discretizations of the Liouville Equation and their Numerical Tests

The main purpose of this article is to show how symmetry structures in par- tial differential equations can be preserved in a discrete world and reflected in difference schemes. Three different structure preserving discretizations of the Liouville equation are presented and then used to solve specific boundary value problems. The results are com- pared with exact solutions satisfying the same boundary conditions. All three discretizations are on four point lattices. One preserves linearizability of the equation, another the infinite- dimensional symmetry group as higher symmetries, the third one preserves the maximal finite-dimensional subgroup of the symmetry group as point symmetries. A 9-point in- variant scheme that gives a better approximation of the equation, but significantly worse numerical results for solutions is presented and discussed.