The special properties of planar posets have been studied, particularly in the 1970's by I. Rival and others. More recently, the connection between posets, their corresponding polynomial rings and corresponding simplicial complexes has been studied by R. Stanley and others. This paper, using work of A. Bjorner, provides a connection between the two bodies of work, by characterizing when planar...

Given a Coxeter system (W,S) equipped with an involutive automorphism θ, the set of twisted identities is ι(θ) = {θ(w)w | w ∈ W}. We point out how ι(θ) shows up in several contexts and prove that if there is no s ∈ S such that sθ(s) is of odd order greater than 1, then the Bruhat order on ι(θ) is a graded poset with rank function ρ given by halving the Coxeter length. Under the same condition, ...

Given a set S of n points in general position, we consider all k-th order Voronoi diagrams on S, for k = 1, . . . , n, simultaneously. We deduce symmetry relations for the number of faces, number of vertices and number of circles of certain orders. These symmetry relations are independent of the position of the sites in S. As a consequence we show that the reduced Euler characteristic of the po...

We recall some abstract connectivity concepts, and apply them to special chains in partially ordered sets, called veins, that are defined as order-convex chains that are contained in every maximal chain they meet. Veins enable us to define a new partial order on the same underlying set, called the pruning order. The associated pruned poset is simpler than the initial poset, but irreducible, coi...

We prove that every height-2 finite poset with three or more points has an incomparable pair (.x, J) such that the proportion of all linear extensions of the poset in which s is less than y is between l/3 and 213. A related result of Koml6s says that the containment interval [l/3,2/3] shrinks to [l/2, l/2] in the limit as the width of height-2 posets becomes large. We conjecture that a poset de...

We introduce a partial order structure on the set of interval orders of a given size, and prove that such a structure is in fact a lattice. We also provide a way to compute meet and join inside this lattice. Finally, we show that, if we restrict to series parallel interval order, what we obtain is the classical Tamari poset.

The absolute order on the hyperoctahedral group Bn is investigated. Using a poset fiber theorem, it is proved that the order ideal of this poset generated by the Coxeter elements is homotopy Cohen–Macaulay. This method results in a new proof of Cohen–Macaulayness of the absolute order on the symmetric group. Moreover, it is shown that every closed interval in the absolute order on Bn is shellab...

For von Neumann algebras M,N not isomorphic to C C and without type I2 summands, we show that for an order-isomorphism f : AbSub M! AbSub N between the posets of abelian von Neumann subalgebras of M and N , there is a unique Jordan ⇤-isomorphism g : M! N with the image g[S] equal to f(S) for each abelian von Neumann subalgebra S of M. The converse also holds. This shows the Jordan structure of ...