This paper describes symmetries of all integrable difference equations that belong to the famous Adler-Bobenko-Suris classification. For each equation, the characteristics of symmetries satisfy a functional equation, which we solve by reducing it to a system of partial differential equations. In this way, all five-point symmetries of integrable equations on the quad-graph are found. These include mastersymmetries, which allow one to construct infinite hierarchies of local symmetries. We also demonstrate a connection between the symmetries of quad-graph equations and those of the corresponding Toda type difference equations.

This paper deals with conservation laws for all integrable difference equations that belong to the famous Adler-Bobenko-Suris classification. All inequivalent three-point conservation laws are found, as are three five-point conservation laws for each equation. We also describe a method of generating conservation laws from known ones; this method can be used to generate higherorder conservation laws from those that are listed here. † O.Rasin@surrey.ac.uk ‡ P.Hydon@surrey.ac.uk

The Bäcklund transformation (BT) for the Camassa-Holm (CH) equation is presented and discussed. Unlike the vast majority of BTs studied in the past, for CH the transformation acts on both the dependent and (one of) the independent variables. Superposition principles are given for the action of double BTs on the variables of the CH and the potential CH equations. Applications of the BT and its superposition principles are presented, specifically the construction of travelling wave solutions, a new method to construct multi-soliton, multi-cuspon and solitoncuspon solutions, and a derivation of generating functions for the local symmetries and conservation laws of the CH hierarchy.

The Gardner method, traditionally used to generate conservation laws of integrable equations, is generalized to generate symmetries. The method is demonstrated for the KdV, Camassa-Holm and Sine-Gordon equations. The method involves identifying a generating symmetry which depends upon a parameter; expansion of this symmetry in a (formal) power series in the parameter then gives the usual infinite hierarchy of symmetries. We show that the obtained symmetries commute, discuss the relation of the Gardner method with Lenard recursion (both for symmetries and conservation laws), and also the connection between the symmetries of continuous integrable equations and their discrete analogs.

This paper presents a systematic investigation of the integrability conditions for nonautonomous quadgraph maps, using the Lax pair approach, the ultra-local singularity confinement criterion and direct construction of conservation laws. We show that the integrability conditions derived from each of the methods are the one and the same, suggesting that there exists a deep connection between these techniques for partial difference equations.