Lie symmetries of difference equations
  The discrete heat equation is worked out in order to illustrate the search of symmetries of difference equations. It is paid an special attention to the Lie structure of these symmetries, as well as to their dependence on the derivative discretization. The case of q-symmetries for discrete equations in a q-lattice is also considered. 
D. Levi, J. Negro and M.A. del Olmo