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Isotropic Schur roots

In this paper, we study the isotropic Schur roots of an acyclic quiver $Q$ with $n$ vertices. We study the perpendicular category $\mathcal{A}(d)$ of a dimension vector $d$ and give a complete description of it when $d$ is an isotropic Schur $δ$. This is done by using exceptional sequences and by defining a subcategory $\mathcal{R}(Q,δ)$ attached to the pair $(Q,δ)$. The latter category is always equivalent to the category of representations of a connected acyclic quiver $Q_{\mathcal{R}}$ of tame type, having a unique isotropic Schur root, say $δ_{\mathcal{R}}$. The understanding of the simple objects in $\mathcal{A}(δ)$ allows us to get a finite set of generators for the ring of semi-invariants SI$(Q,δ)$ of $Q$ of dimension vector $δ$. The relations among these generators come from the representation theory of the category $\mathcal{R}(Q,δ)$ and from a beautiful description of the cone of dimension vectors of $\mathcal{A}(δ)$. Indeed, we show that SI$(Q,δ)$ is isomorphic to the ring of semi-invariants SI$(Q_{\mathcal{R}},δ_{\mathcal{R}})$ to which we adjoin variables. In particular, using a result of Skowroński and Weyman, the ring SI$(Q,δ)$ is a polynomial ring or a hypersurface.