Algebraic Structure of Discrete Zero Curvature Equations and Master   Symmetries of Discrete Evolution Equations
  An algebraic structure related to discrete zero curvature equations is established. It is used to give an approach for generating master symmetries of first degree for systems of discrete evolution equations and an answer to why there exist such master symmetries. The key of the theory is to generate nonisospectral flows $(\lambda_t=\lambda ^l, l\ge0)$ from the discrete spectral problem associated with a given system of discrete evolution equations. Three examples are given. 
Wen-Xiu Ma and Benno Fuchssteiner