Preorders on maximal chains: hyperplane arrangements, Cambrian lattices, and maximal green sequences

Abstract: We study preorders on (equivalence classes of) maximal chains in the general context of polygonal lattices endowed with suitably nice edge labellings. We show that, given a quotient of polygonal lattices, such edge labellings descend to the quotient, and that there is an induced order-preserving surjective map on the preordered sets of equivalence classes of maximal chains. Under a natural condition ensuring that the domain is a poset, the map is a contraction of preordered sets. We apply this to lattices of regions of simplicial hyperplane arrangements, where the preorders are partial orders, in particular to finite Coxeter arrangements. For the latter, each choice of Coxeter element gives us a different partial order on the set of equivalence classes of maximal chains; these generalise certain reoriented higher Bruhat orders in dimension two. The maps of posets of maximal chains induced by Cambrian congruences generalise the map of Kapranov and Voevodsky from the higher Bruhat orders to the higher Stasheff--Tamari orders in dimension two. While the fibres of this map are known not always to be intervals, our results show that they are always connected. We show that, in the case of Cambrian lattices, the induced maps on maximal chains have nice descriptions in terms of orientations of rank-two root subsystems, in a way which resembles taking the stable objects of a Rudakov stability condition. We finally consider the algebraic realisation of the weak order and Cambrian lattices via torsion-free classes of preprojective and path algebras, relating the posets of maximal chains to our earlier work on maximal green sequences.