Bricks over preprojective algebras and join-irreducible elements in Coxeter groups

Abstract: A (semi)brick over an algebra $A$ is a module $S$ such that the endomorphism ring $\operatorname{\mathsf{End}}A(S)$ is a (product of) division algebra. For each Dynkin diagram $\Delta$, there is a bijection from the Coxeter group $W$ of type $\Delta$ to the set of semibricks over the preprojective algebra $\Pi$ of type $\Delta$, which is restricted to a bijection from the set of join-irreducible elements of $W$ to the set of bricks over $\Pi$. This paper is devoted to giving an explicit description of these bijections in the case $\Delta=\mathbb{A}_n$ or $\mathbb{D}_n$. First, for each join-irreducible element $w \in W$, we describe the corresponding brick $S(w)$ in terms of "Young diagram-like" notation. Next, we determine the canonical join representation $w=\bigvee{i=1}^m w_i$ of an arbitrary element $w \in W$ based on Reading's work, and prove that $\bigoplus_{i=1}^n S(w_i)$ is the semibrick corresponding to $w$.