Expansion Posets for Polygon Cluster Algebras

Abstract

Define an expansion poset to be the poset of monomials of a cluster variable attached to an arc in a polygon, where each monomial is represented by the corresponding combinatorial object from some fixed combinatorial cluster expansion formula. We introduce an involution on several of the interrelated combinatorial objects and constructions associated to type $A$ surface cluster algebras, including certain classes of arcs, triangulations, and distributive lattices. We use these involutions to formulate a dual version of skein relations for arcs, and dual versions of three existing expansion posets. In particular, this leads to two new cluster expansion formulas, and recovers the lattice path expansion of Propp et al. We provide an explicit, structure-preserving poset isomorphism between an expansion poset and its dual version from the dual arc. We also show that an expansion poset and its dual version constructed from the same arc are dual in the sense of distributive lattices. We show that any expansion poset is isomorphic to a closed interval in one of the lattices $L(m,n)$ of Young diagrams contained in an $m \times n$ grid, and that any $L(m,n)$ has a covering by such intervals. We give two formulas for the rank function of any lattice path expansion poset, and prove that this rank function is unimodal whenever the underlying snake graph is built from at most four maximal straight segments. We show that the support of any type $A$ cluster variable is the orbit of a groupoid. Finally, in work joint with Nicholas Ovenhouse, we partially generalize $T$-paths to configurations of affine flags, and prove that a $T$-path expansion analogous to the type $A$ case holds when the initial seed is from a fan triangulation.