# Question about unusual highest weight modules for $U_q(sl(2))$

Background

Let $U_q(sl(2))$ be the quantum group associated with $sl(2)$ i.e. the associative algebra with 1 over $Q(q)$ generated by $x^+,x^-,K,K^{-1}$ with relations $$KK^{-1}=K^{-1}K=1$$ $$Kx^+K^{-1}=q^2x^+,Kx^-K^{-1}=q^{-2}x^-$$ $$x^+x^{-}-x^{-}x^+=\frac{K-K^{-1}}{q-q^{-1}}$$ Here $q$ is indefinite, in particular not a root of unity. A $U_q(sl(2))$-module $V$ has highest weight $\omega \in Q(q)$ if there exists a vector $v \in V$ such that $$U_q(sl(2))\cdot v=V$$ $$x^+\cdot v=0$$ $$K\cdot v=\omega v$$ Most standard texts (e.g. Chari Pressley, etc.) focus mainly on the case where $\omega$ equals $q$ to some power (i.e. $K$ acts as $q^m$ for some integer $m$ or $-q^m$ though this case is equivalent to the first by tensoring with a 1-dimensional module). If $\omega$ is not of this form then the module in question is necessarily infinite dimensional.

Question

Has anyone considered what happens when $K$ acts by something other than a power of $q$, say $2q-1$? I am especially interested in what happens in the $q=1$ specialization of these modules. Any references would be appreciated.

When talking about modules for such a quantized enveloping algebra, you have to be explicit about whether the module is finite dimensional or not. (For affine or other infinite dimensional Lie algebras of interest, the parallel question is whether modules are integrable or not.) In Jantzen's introductory AMS text Lectures on Quantum Groups, for instance, he starts out with analogues of the traditional finite dimensional representations. Here the associated highest weights are (up to sign) given by powers of $q$ in the rank one case. The infinite dimensional modules with dominant integral highest weights then have reasonable properties as well.
• @Evan: There is a lot of literature on "quantum groups", but for instance one recent paper by H.H. Andersen and v. Mazorchuk develops some of the good analogies with category $\mathcal{O}$ for certain quantum groups: front.math.ucdavis.edu/1105.5500 – Jim Humphreys Feb 21 '13 at 12:36