Crystals of Lakshmibai-Seshadri paths and extremal weight modules over quantum hyperbolic Kac-Moody algebras of rank 2

Abstract

Let $\mathfrak{g}$ be a hyperbolic Kac-Moody algebra of rank $2$, and let $\lambda$ be an arbitrary integral weight. We denote by $\mathbb{B}(\lambda)$ the crystal of all Lakshmibai-Seshadri paths of shape $\lambda$. Let $V(\lambda)$ be the extremal weight module of extremal weight $\lambda$ generated by the (cyclic) extremal weight vector $v_\lambda$ of weight $\lambda$, and let $\mathcal{B}(\lambda)$ be the crystal basis of $V(\lambda)$ with $u_\lambda \in \mathcal{B}(\lambda)$ the element corresponding to $v_\lambda$. We prove that the connected component $\mathcal{B}_0(\lambda)$ of $\mathcal{B}(\lambda)$ containing $u_\lambda$ is isomorphic, as a crystal, to the connected component $\mathbb{B}_0(\lambda)$ of $\mathbb{B}(\lambda)$ containing the straight line $\pi_\lambda$. Furthermore, we prove that if $\lambda$ satisfies a special condition, then the crystal basis $\mathcal{B}(\lambda)$ is isomorphic, as a crystal, to the crystal $\mathbb{B}(\lambda)$. As an application of these results, we obtain an algorithm for computing the number of elements of weight $\mu$ in $\mathcal{B}(\Lambda_1-\Lambda_2)$, where $\Lambda_1, \Lambda_2$ are the fundamental weights, in the case that $\mathfrak{g}$ is symmetric.