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arXiv:solv-int/9704015 · Modern Physics Letters A

Fusion Rules for Quantum Transfer Matrices as a Dynamical System on Grassmann Manifolds

O. Lipan, Paul B. Wiegmann, A. Zabrodin

Published Jun 21, 1997• 16 citations
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Abstract

We show that the set of transfer matrices of an arbitrary fusion type for an integrable quantum model obeys these bilinear functional relations, which are identified with an integrable dynamical system on a Grassmann manifold (higher Hirota equation). The bilinear relations were previously known for a particular class of transfer matrices corresponding to rectangular Young diagrams. We extend this result for general Young diagrams. A general solution of the bilinear equations is presented.

DOI
10.1142/s0217732397001394

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We show that the set of transfer matrices of an arbitrary fusion type for an integrable quantum model obeys these bilinear functional relations, which are identified with an integrable dynamical system on a Grassmann manifold (higher Hirota equation). The bilinear relations were previously known for a particular class of transfer matrices corresponding to rectangular Young diagrams. We extend this result for general Young diagrams. A general solution of the bilinear equations is presented.
The recent progress in revealing classical integrable structures in quantum models solved by Bethe ansatz is reviewed. Fusion relations for eigenvalues of quantum transfer matrices can be written in the form of classical Hirota's bilinear difference equation. This equation is also known as the completely discretized version of the 2D Toda lattice. We explain how one obtains the specific quantum results by solving the classical equation. The auxiliary linear problem for the Hirota equation is shown to generalize Baxter's T-Q relation.
Abstract For an arbitrary generalized quantum integrable spin chain we introduce a “masterT-operator” which represents a generating function for commuting quantum transfer matrices constructed by means of the fusion procedure in the auxiliary space. We show that the functional relations for the transfer matrices are equivalent to an infinite set of model-independent bilinear equations of the Hirota form for the masterT-operator, which allows one to identify it withτ-function of an integrable hierarchy of classical soliton equations. In this paper we consider spin chains with rationalGL(N)-invariantR-matrices but the result is independent of a particular functional form of the transfer matrices and directly applies to quantum integrable models with more general (trigonometric and elliptic)R-matrices and to supersymmetric spin chains.
The recent progress in revealing classical integrable structures in quantum models solved by Bethe ansatz is reviewed. Fusion relations for eigenvalues of quantum transfer matrices can be written in the form of classical Hirota’s bilinear difference equation. This equation is also known as the completely discretized version of the 2D Toda lattice. We explain how one obtains the specific quantum results by solving the classical equation. The auxiliary linear problem for the Hirota equation is shown to generalize Baxter’s T-Q relation.
In this paper, we will construct the connection between the quantum Gaudin model and the universal character hierarchy by a newly defined coupled master T-operator. The coupled master T-operator of the quantum Gaudin model satisfies the bilinear identity of the universal character hierarchy, which is an extension of the Kadomtsev–Petviashvili hierarchy. In addition, for the generalized quantum integrable spin chains with rational GL(N)-invariant R-matrices, we construct its coupled master T-operator, which represents a generating function for two-folds commuting quantum transfer matrices. The functional relations for the transfer matrices are equivalent to an infinite set of Hirota bilinear equations of the modified universal character hierarchy.
This chapter focuses on the derivation and interpretation of functional relations among commuting transfer matrices, which underpin a wide range of powerful techniques in quantum integrable systems. These include spectral equations, Baxter’s TQ-relations, and the analytic Bethe ansatz, as well as methods based on wronskian Bethe equations. Rather than isolated tools, these structures are intricately connected and reveal deep algebraic insights with significant implications, including quantum–classical duality and the completeness problem. Further emphasis is placed on the group-theoretic structure underlying these functional relations. Connections are drawn with the classical theory of characters, where objects such as Schur polynomials and Jacobi–Trudi formulae lead to bilinear relations naturally interpreted as Hirota equations in the context of transfer matrices. This perspective offers a conceptual bridge between quantum integrable models and classical representation theory, highlighting the unifying role of functional relations in both structure and application. Special attention is given to Q-functions governed by Baxter’s TQ-relations and, more broadly, by determinant identities from the wronskian formalism. These functions encode the transfer matrix eigenvalues and serve as fundamental algebraic objects. Functional methods thus offer powerful tools for exploring integrable models, especially when conventional Bethe ansatz techniques become impractical.