On finite-dimensional multiplicity-free irreducible modules for a nil-DAHA of type $(C_1^\vee,C_1)$

Abstract

Fix nonzero $r_0,r_1\in\mathbb{C}$. Let $\widetilde{\mathcal H}$ denote a nil-DAHA of type $(C_1^\vee,C_1)$ defined by generators $t_0,u_0,t_1,u_1$ and relations $(t_i-r_i)(t_i-r_i^{-1})=0$ for $i\in\{0,1\}$, $u_0^2=u_0$, $u_1^2=0$, and $u_0t_0t_1u_1=0=t_1u_1u_0t_0$. Set $A=u_0t_0$ and $B=t_1u_1$. A finite-dimensional $\widetilde{\mathcal H}$-module is called $(A,B)$-multiplicity-free, or simply multiplicity-free, if $A$ and $B$ are simultaneously diagonalizable and every nonzero common eigenspace is one-dimensional. We consider finite-dimensional irreducible multiplicity-free modules that have a certain ordered basis, which we call an adapted block basis. For $D\geq 1$, we construct a family of $2D$-dimensional $\widetilde{\mathcal H}$-modules $E_D$, and for $D\geq 0$, we construct a family of $(2D+1)$-dimensional $\widetilde{\mathcal H}$-modules $O_D$. We determine which members of these families are multiplicity-free and irreducible. We prove that every finite-dimensional irreducible $\widetilde{\mathcal H}$-module that is multiplicity-free and has an adapted block basis is isomorphic to a module of the form $E_D$ or $O_D$. We also determine when two members of the same family are isomorphic.