The Whitney Duals of a Graded Poset

TL;DR

This paper introduces Whitney duals of graded posets, providing a construction method, exploring their properties, and demonstrating their existence for various important classes of posets, with applications in combinatorics and algebra.

Contribution

It defines Whitney duals and Whitney labelings, proves their existence for posets with Whitney labelings, and applies these concepts to several well-studied posets, including geometric lattices and noncrossing partitions.

Findings

Every graded poset with a Whitney labeling has a Whitney dual.

Explicit construction of Whitney duals using quotient posets.

Applications to geometric lattices, noncrossing partitions, and weighted partitions.

Abstract

We introduce the notion of a \emph{Whitney dual} of a graded poset. Two posets are Whitney duals to each other if (the absolute value of) their Whitney numbers of the first and second kind are interchanged between the two posets. We define new types of edge and chain-edge labelings which we call \emph{Whitney labelings}. We prove that every graded poset with a Whitney labeling has a Whitney dual. Moreover, we show how to explicitly construct a Whitney dual using a technique involving quotient posets. As applications of our main theorem, we show that geometric lattices, the lattice of noncrossing partitions, the poset of weighted partitions studied by Gonz\'alez D'Le\'on-Wachs, and most of the R$^*$S-labelable posets studied by Simion-Stanley all have Whitney duals. Our technique gives a combinatorial description of a Whitney dual of the noncrossing partition lattice in terms of a family of noncrossing Dyck paths. Our method also provides an explanation of the Whitney duality between the poset of weighted partitions and a poset of rooted forests studied by Reiner and Sagan. An integral part of this explanation is a new chain-edge labeling for the poset of weighted partitions which we show is a Whitney labeling. Finally, we show that a graded poset with a Whitney labeling admits a local action of the $0$-Hecke algebra of type $A$ on its set of maximal chains. The characteristic of the associated representation is Ehrenborg's flag quasisymmetric function. The existence of this action implies, using a result of McNamara, that when the maximal intervals of the constructed Whitney duals are bowtie-free, they are also snellable. In the case where these maximal intervals are lattices, they are supersolvable.