Symmetries, Conservation Laws, and Noether's Theorem for Differential-Difference Equations

This paper mainly contributes to the extension of Noether's theorem to differential-difference equations. For this purpose, we first investigate the prolongation formula for continuous symmetries, which makes a characteristic representation possible. The relations of symmetries, conservation laws, and the Fréchet derivative are also investigated. For nonvariational equations, because Noether's theorem is now available, the self-adjointness method is adapted to the computation of conservation laws for differential-difference equations. Several differential-difference equations are investigated as illustrative examples, including the Toda lattice and semidiscretizations of the Korteweg-de Vries (KdV) equation. In particular, the Volterra equation is taken as a running example.