Asymmetric Integrable Quad-graph Equations

Integrable difference equations commonly have more low-order conservation laws than occur for non-integrable difference equations of similar complexity. We use this empirical observation to sift a large class of difference equations, in order to find candidates for integrability. It turns out that all such candidates have an equivalent affine form. These are tested by calculating their algebraic entropy. In this way, we have found several types of integrable equations, two of which seem to be new. One of the new equations occurs as a singular limit of the lattice MKdV equation; the remaining one seems to be isolated from all currently-known discrete integrable systems. We also list all single-tile conservation laws for the integrable equations in the above class.