Irreducible Modules for the Quantum Affine Algebra $U_q(\hat{sl}_2)$ and its Borel subalgebra $U_q(\hat{sl}_2)^{\geq 0}$

Abstract

Let $U_q(\hat{sl}_2)^{\geq 0}$ denote the Borel subalgebra of the quantum affine algebra $U_q(\hat{sl}_2)$. We show that the following hold for any choice of scalars $\epsilon_0, \epsilon_1$ from the set ${1,-1}$. (i) Let $V$ be a finite-dimensional irreducible $U_q(\hat{sl}_2)^{\geq 0}$-module of type $(\epsilon_0,\epsilon_1)$. Then the action of $U_q(\hat{sl}_2)^{\geq 0}$ on $V$ extends uniquely to an action of $U_q(\hat{sl}_2)$ on $V$. The resulting $U_q(\hat{sl}_2)$-module structure on $V$ is irreducible and of type $(\epsilon_0,\epsilon_1)$. (ii) Let $V$ be a finite-dimensional irreducible $U_q(\hat{sl}_2)$-module of type $(\epsilon_0,\epsilon_1)$. When the $U_q(\hat{sl}_2)$-action is restricted to $U_q(\hat{sl}_2)^{\geq 0}$, the resulting $U_q(\hat{sl}_2)^{\geq 0}$-module structure on $V$ is irreducible and of type $(\epsilon_0,\epsilon_1)$.