S. Carpentier, A.V. Mikhailov, and Jing Ping Wang. PreHamiltonian and Hamiltonian operators for differential-difference equations, Nonlinearity 33 915 (2020).

## Abstract

In this paper we are developing a theory of rational (pseudo) difference Hamiltonian operators, focusing in particular on its algebraic aspects. We show that a pseudo-difference Hamiltonian operator can be represented as a ratio *AB-1* of two difference operators with coefficients from a difference field F, where *A* is preHamiltonian. A difference operator *A* is called preHamiltonian if its image is a Lie subalgebra with respect to the Lie bracket of evolutionary vector fields on F. The definition of a rational Hamiltonian operator can be reformulated in terms of its factors which simplifies the theory and makes it useful for applications. In particular we show that for a given rational Hamiltonian operator *H* in order to find a second Hamiltonian operator *K *compatible with *H *one only needs to find a preHamiltonian pair *A* and *B* such that *K* = *AB* −1*H* is skew-symmetric. We apply our theory to study multi-Hamiltonian structures of Narita–Itoh–Bogayavlensky and Adler–Postnikov equations.