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

We introduce hierarchies of difference equations (referred to as $nT$-systems) associated to the action of a (centrally extended, completed) infinite matrix group $G{L}_{\mathrm{\infty }}^{(n)}$ 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 $G{L}_{\mathrm{\infty }}^{(n)}$ to a subgroup isomorphic to the loop group $LG{L}_n$, defines $nQ$-systems, studied earlier by the present authors for $n=2,3$.