Difference Hierarchies for $nT$ $\tau$-Functions

TL;DR

This paper introduces hierarchies of difference equations called $nT$-systems linked to an infinite matrix group acting on fermionic Fock space, revealing new bilinear relations and connections to known $T$-systems.

Contribution

It defines $nT$-hierarchies of difference equations, relates them to $ au$-functions, and connects these to classical $T$-systems and $Q$-systems for the first time.

Findings

$nT$-systems satisfy bilinear equations of length 3 to $n+1$

The $2T$-system reduces to the classical $A$-type $3$-term $T$-system

Restriction to loop groups yields $nQ$-systems previously studied by authors

Abstract

We introduce hierarchies of difference equations (referred to as $nT$-systems) associated to the action of a (centrally extended, completed) infinite matrix group $GL_{\infty}^{(n)}$ on $n$-component fermionic Fock space. The solutions are given by matrix elements ($\tau$-functions) for this action. We show that the $\tau$-functions of type $nT$ satisfy bilinear equations of length $3,4,\dots,n+1$. The $2T$-system is, after a change of variables, the usual $3$ term $T$-system of type $A$. Restriction from $GL_{\infty}^{(n)}$ to a subgroup isomorphic to the loop group $LGL_{n}$, defines $nQ$-systems, studied earlier by the present authors for $n=2,3$.