Abstract. We classify the simple infinite dimensional integrable modules with finite dimensional weight spaces over the quantized enveloping algebra of an untwisted affine algebra. We prove that these are either highest (lowest) weight integrable modules or simple submodules of a loop module of a finitedimensional simple integrable module and describe the latter class. Their characters and crystal bases theory are discussed in a special case.
IntroductionThe aim of the present paper is to study irreducible integrable modules for quantum affine algebras, which have finite dimensional weight spaces. The best known examples of such representations are the highest weight representations V (λ) (cf. [16, 23]) on which the center of the quantum affine algebra acts via a positive integer power of q. These representations have many pleasant properties, for instance it is known that they admit a canonical global or crystal bases. Another family of integrable modules for the quantum affine algebra are the finite-dimensional modules which have been studied, amongst the others, in [1, 8,11,12,13, 18,24, 25,26]. However, unlike the highest weight representations, these finite-dimensional representations do not respect the natural Z-grading on the quantum affine algebra which arises from the adjoint action of the element of the torus corresponding to the Euler operator. Thus it is natural to look for the graded analogue of the finite-dimensional modules. Besides, in certain cases one has to consider these infinite dimensional modules instead of finite-dimensional ones. For example, a finite-dimensional module cannot appear as a submodule of the ring of linear endomorphisms of V (λ) whilst a simple integrable module with non-trivial zero weight space can be embedded in such a ring for λ sufficiently large (cf. for example [20]).Examples of infinite dimensional integrable modules which are not highest weight modules are easy to construct. Namely, given a finite-dimensional module V , one can define on the space L(V ) = V ⊗ C(q)[t, t −1 ] in an obvious way the structure of a graded module for the quantum affine algebra. However, even if V is irreducible, the resulting representation L(V ) need not remain so.The irreducible finite-dimensional representations of the quantum affine algebra are known to be parametrized by families of polynomials in an indeterminate u