Tau-functions beyond the group elements

Matrix elements in different representations are connected by quadratic relations. If matrix elements are those of a group element, i.e. satisfying the property Δ(X)=X⊗X, then their generating functions obey bilinear Hirota equations and hence are named τ-functions. However, dealing with group elements is not always easy, especially for non-commutative algebras of functions, and this slows down the development of τ-function theory and the study of integrability properties of non-perturbative functional integrals. A simple way out is to use arbitrary elements of the universal enveloping algebra, and not just the group elements. Then the Hirota equations appear to interrelate a whole system of generating functions, which one may call generalized τ-functions. It was recently demonstrated that this idea can be applicable even to a somewhat sophisticated case of the quantum toroidal algebra. We consider a number of simpler examples, including ordinary and quantum groups, to explain how the method works and what kind of solutions one can obtain.