Auslander algebras, flag combinatorics and quantum flag varieties

Let D be the Auslander algebra of C[t]/(tⁿ), which is quasi-hereditary, and F_Δ the subcategory of good D-modules. For any J⊆[1, n-1], we construct a subcategory F_Δ(J) of F_Δ with an exact structure E. We show that under E, F_Δ(J) is Frobenius stably 2-Calabi-Yau and admits a cluster structure consisting of cluster tilting objects. This then leads to an additive categorification of the cluster structure on the coordinate ring C[Fl(J)] of the (partial) flag variety Fl(J). We further apply F_Δ(J) to study flag combinatorics and the quantum cluster structure on the flag variety Fl(J). We show that weak and strong separation can be detected by the extension groups ext¹(-, -) under E and the extension groups Ext¹(-,-), respectively. We give a interpretation of the quasi-commutation rules of quantum minors and identify when the product of two quantum minors is invariant under the bar involution. The combinatorial operations of flips and geometric exchanges correspond to certain mutations of cluster tilting objects in F_Δ(J). We then deduce that any (quantum) minor is reachable, when J is an interval. Building on our result for the interval case, Geiss-Leclerc-Schr\"{o}er's result on the quantum coordinate ring for the open cell of Fl(J) and Kang-Kashiwara-Kim-Oh's enhancement of that to the integral form, we prove that Cq[Fl(J)] is a quantum cluster algebra over C[q,q⁻¹].