Stability, shards, and preprojective algebras

The goal of this note two-fold. First, I draw attention to the way that semistability (in the sense of King) gives us a geometrical picture of (some of) the extension-closed abelian subcategories of a finite-dimensional algebra. Second, I describe Nathan Reading’s shards of a hyperplane arrangement, and explain their relevance to understanding the semistability picture for finite-type preprojective algebras.