Noncrossing Sets and a Grassmann Associahedron

We study a natural generalization of the noncrossing relation between pairs of elements in [n] to k-tuples in [n] that was first considered by Petersen et al. [J. Algebra 324(5) (2010), 951–969]. We give an alternative approach to their result that the flag simplicial complex on ([n] k ) induced by this relation is a regular, unimodular and flag triangulation of the order polytope of the poset given by the product [k] × [n − k] of two chains (also called Gelfand–Tsetlin polytope), and that it is the join of a simplex and a sphere (that is, it is a Gorenstein triangulation). We then observe that this already implies the existence of a flag simplicial polytope generalizing the dual associahedron, whose Stanley–Reisner ideal is an initial ideal of the Grassmann–Plücker ideal, while previous constructions of such a polytope did not guarantee flagness nor reduced to the dual associahedron for k = 2. On our way we provide general results about order polytopes and their triangulations. We call the simplicial complex the noncrossing complex, and the polytope derived from it the dual Grassmann associahedron. We extend results of Petersen et al. [J. Algebra 324(5) (2010), 951–969] showing that the noncrossing complex and the Grassmann associahedron naturally reflect the relations between Grassmannians with different parameters, in particular the isomorphism Gk,n ∼= Gn−k,n . Moreover, our approach allows us to show that the adjacency graph of the noncrossing complex admits a natural acyclic orientation that allows us to define a Grassmann– Tamari order on maximal noncrossing families. Finally, we look at the precise relation of the noncrossing complex and the weak separability complex of Leclerc and Zelevinsky [Amer. Math. Soc. Transl. 181(2) (1998), 85–108]; see also Scott [J. Algebra 290(1) (2005), 204–220] among others. We show that the weak separability complex is not only a subcomplex of the noncrossing c © The Author(s) 2017. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/fms.2017.1 Downloaded from https:/www.cambridge.org/core. IP address: 54.200.109.4, on 25 Feb 2017 at 12:59:32, subject to the Cambridge Core terms F. Santos, C. Stump and V. Welker 2 complex as noted by Petersen et al. [J. Algebra 324(5) (2010), 951–969] but actually its cyclically invariant part. 2010 Mathematics Subject Classification: 52B20 (primary); 06A11 (secondary)