Linear integrable systems on quad-graphs

TL;DR

This paper classifies linear integrable quad-equations on bipartite isoradial quad-graphs, constructs solutions using elliptic functions, and demonstrates the integrability of related discrete variational problems through multi-dimensional consistency.

Contribution

It provides a complete classification of certain integrable quad-equations and introduces a new two-field generalization of the star-triangle map with proven integrability.

Findings

Classification reduces to solving a functional equation.

Explicit solutions expressed in elliptic functions.

Proven multi-dimensional consistency of the new 3D system.

Abstract

In the first part of the paper, we classify linear integrable (multi-dimensionally consistent) quad-equations on bipartite isoradial quad-graphs in $\mathbb C$, enjoying natural symmetries and the property that the restriction of their solutions to the black vertices satisfies a Laplace type equation. The classification reduces to solving a functional equation. Under certain restriction, we give a complete solution of the functional equation, which is expressed in terms of elliptic functions. We find two real analytic reductions, corresponding to the cases when the underlying complex torus is of a rectangular type or of a rhombic type. The solution corresponding to the rectangular type was previously found by Boutillier, de Tili\`ere and Raschel. Using the multi-dimensional consistency, we construct the discrete exponential function, which serves as a basis of solutions of the quad-equation. In the second part of the paper, we focus on the integrability of discrete linear variational problems. We consider discrete pluri-harmonic functions, corresponding to a discrete 2-form with a quadratic dependence on the fields at black vertices only. In an important particular case, we show that the problem reduces to a two-field generalization of the classical star-triangle map. We prove the integrability of this novel 3D system by showing its multi-dimensional consistency. The Laplacians from the first part come as a special solution of the two-field star-triangle map.