Branching rules for level-zero extremal weight modules from $U_q(\widehat{\mathfrak{sl}}_{n+1})$ to $U_q(\widehat{\mathfrak{sl}}_n)$

Abstract

In this paper, we study the structure of a $U_q(\widehat{\mathfrak{sl}}_n)$-module $\Psi_{\varepsilon}^* V(\lambda)$, where $V(\lambda)$ is the extremal weight module of level-zero dominant weight $\lambda$ over the quantum affine algebra $U_q(\widehat{\mathfrak{sl}}_{n+1})$ and $\Psi_{\varepsilon}: U_q(\widehat{\mathfrak{sl}}_n) \to U_q(\widehat{\mathfrak{sl}}_{n+1})$ is an injective algebra homomorphism. We establish a direct sum decomposition $\Psi_{\varepsilon}^* V(\lambda) \cong M_{0,\varepsilon} \oplus \cdots \oplus M_{m,\varepsilon}$, where $M_{0,\varepsilon}$ and $M_{m,\varepsilon}$ are isomorphic to a tensor product of an extremal weight module over $U_q(\widehat{\mathfrak{sl}}_n)$ and a symmetric Laurent polynomial ring. Moreover, when $\lambda$ is a multiple of a level-zero fundamental weight, we show that $\Psi_{\varepsilon}^* V(\lambda)$ is isomorphic to a direct sum of extremal weight modules.