Verma modules and finite-dimensional irreducible modules of the universal Askey--Wilson algebra at roots of unity

Abstract: Assume that $\mathbb F$ is an algebraically closed field and fix a nonzero scalar $q\in \mathbb F$ with $q^4\not=1$. The universal Askey--Wilson algebra $\triangle_q$ is a unital associative algebra over $\mathbb F$ defined by generators and relations. The generators are $A,B,C$ and the relations assert that each of \begin{gather*} A+\frac{qBC-q^{-1}CB}{q^2-q^{-2}}, \qquad B+\frac{qCA-q^{-1}AC}{q^2-q^{-2}}, \qquad C+\frac{qAB-q^{-1}BA}{q^2-q^{-2}} \end{gather*} commutes with $A,B,C$. The Verma $\triangle_q$-modules are a family of infinite-dimensional $\triangle_q$-modules with marginal weights. Under the condition that $q$ is not a root of unity, it was shown that every finite-dimensional irreducible $\triangle_q$-module has a marginal weight and is isomorphic to a quotient of a Verma $\triangle_q$-module. Assume that $q$ is a root of unity. We prove that every finite-dimensional irreducible $\triangle_q$-module with a marginal weight is isomorphic to a quotient of a Verma $\triangle_q$-module. Properly speaking, two natural families of finite-dimensional quotients of Verma $\triangle_q$-modules contain all finite-dimensional irreducible $\triangle_q$-modules with marginal weights up to isomorphism. Furthermore, we classify the finite-dimensional irreducible $\triangle_q$-modules with marginal weights up to isomorphism.