Published online by Cambridge University Press: 05 July 2011
Introduction
In this chapter we discuss the application of generalized symmetries to the investigation of difference and differential-difference equations. This is a sequel to the presentation of P. Winternitz where Lie point symmetries for difference equations have been introduced and studied in detail. In particular it has been shown there that for a given discrete equation, unless we allow for variable lattices, i.e., we consider a difference scheme, very few symmetries are present. So, if we want to get symmetries for difference equations, either we consider the point symmetries of a difference scheme or we extend the class of symmetries to the case of the generalized symmetries. In the following we will proceed in this second direction and analyze the structure of the generalized symmetries for a difference equation. We will limit ourselves to consider just partial difference equations (with two independent variables) where the lattice is fixed and non-transformable and either all independent variables are discrete (n,m) or one is discrete n and one is continuous t. We will limit our discussion to the case of scalar equations of a low order, i.e., when the dependent variable is a scalar and the differential difference equations involve at most derivatives of the second order of the fields and nearest neighboring interactions.
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