On a class of 2D integrable lattice equations

We develop a new approach to the classification of integrable equations of the form u_xy = f(u, u_x, u_y, Δ_zu Δ_z¯u, Δ _zz¯u) where Δ_z and Δ_z¯ are the forward/backward discrete derivatives. The following 2-step classification procedure is proposed: (1) First we require that the dispersionless limit of the equation is integrable, that is, its characteristic variety defines a conformal structure which is Einstein-Weyl on every solution. (2) Secondly, to the candidate equations selected at the previous step we apply the test of Darboux integrability of reductions obtained by imposing suitable cut-off conditions