Lakshmibai-Seshadri paths for hyperbolic Kac-Moody algebras of rank $2$

Abstract

Let $\mathfrak{g}$ be a hyperbolic Kac-Moody algebra of rank $2$, and set $\lambda: = \Lambda_1 - \Lambda_2$, where $\Lambda_1, \Lambda_2$ are the fundamental weights for $\mathfrak{g}$; note that $\lambda$ is neither dominant nor antidominant. Let $\mathbb{B}(\lambda)$ be the crystal of all Lakshmibai-Seshadri paths of shape $\lambda$. We prove that (the crystal graph of) $\mathbb{B}(\lambda)$ is connected. Furthermore, we give an explicit description of Lakshmibai-Seshadri paths of shape $\lambda$.