Method for searching higher symmetries for quad graph equations
  Generalized symmetry integrability test for discrete equations on the square lattice is studied. Integrability conditions are discussed. A method for searching higher symmetries (including non-autonomous ones) for quad graph equations is suggested based on characteristic vector fields. 
Rustem N. Garifullin, Elena V. Gudkova and Ismagil T. Habibullin
nlin.SI