Orthogonal Laurent polynomials in unit circle, extended CMV ordering and 2D Toda type integrable hierarchies

TL;DR

This paper develops a comprehensive algebraic framework connecting orthogonal Laurent polynomials on the unit circle with Toda-type integrable systems, introducing extended recursions, deformations, and tau-function representations.

Contribution

It extends the theory of orthogonal Laurent polynomials on the unit circle, linking them to integrable hierarchies and providing new algebraic and deformation-based insights.

Findings

Derived explicit recursion relations and Christoffel-Darboux formulas.

Connected orthogonal Laurent polynomials to Toda-type integrable systems.

Presented tau-function formalism and bilinear equations for these polynomials.

Abstract

Orthogonal Laurent polynomials in the unit circle and the theory of Toda-like integrable systems are connected using the Gauss--Borel factorization of a Cantero-Moral-Velazquez moment matrix, which is constructed in terms of a complex quasi-definite measure supported in the unit circle. The factorization of the moment matrix leads to orthogonal Laurent polynomials in the unit circle and the corresponding second kind functions. Jacobi operators, 5-term recursion relations and Christoffel-Darboux kernels, projecting to particular spaces of truncated Laurent polynomials, and corresponding Christoffel-Darboux formulae are obtained within this point of view in a completely algebraic way. Cantero-Moral-Velazquez sequence of Laurent monomials is generalized and recursion relations, Christoffel-Darboux kernels, projecting to general spaces of truncated Laurent polynomials and corresponding Christoffel-Darboux formulae are found in this extended context. Continuous deformations of the moment matrix are introduced and is shown how they induce a time dependant orthogonality problem related to a Toda-type integrable system, which is connected with the well known Toeplitz lattice. Using the classical integrability theory tools the Lax and Zakharov-Shabat equations are obtained. The dynamical system associated with the coefficients of the orthogonal Laurent polynomials is explicitly derived and compared with the classical Toeplitz lattice dynamical system for the Verblunsky coefficients of Szeg\H{o} polynomials for a positive measure. Discrete flows are introduced and related to Darboux transformations. Finally, the representation of the orthogonal Laurent polynomials (and its second kind functions), using the formalism of Miwa shifts, in terms of $\tau$-functions is presented and bilinear equations are derived.