Poset modules of the $0$-Hecke algebras and related quasisymmetric power sum expansions

TL;DR

This paper explores the structure of poset modules over 0-Hecke algebras, connecting them to quasisymmetric functions and providing new algorithms and formulas for power sum expansions related to Schur functions and other symmetric functions.

Contribution

It establishes a Hopf algebra structure on Grothendieck groups of poset modules, links these modules to quasisymmetric functions, and introduces algorithms for poset construction and power sum expansions.

Findings

The Grothendieck group forms a Hopf algebra isomorphic to quasisymmetric functions.

Poset modules can be used to express quasisymmetric power sums of certain functions.

New algorithms and formulas for power sum expansions involving border strip tableaux.

Abstract

Duchamp--Hivert--Thibon introduced the construction of a right $H_n(0)$-module, denoted as $M_P$, for any partial order $P$ on the set $[n]$. This module is defined by specifying a suitable action of $H_n(0)$ on the set of linear extensions of $P$. In this paper, we refer to this module as the poset module associated with $P$. Firstly, we show that $\bigoplus_{n \ge 0} G_0(\mathscr{P}(n))$ has a Hopf algebra structure that is isomorphic to the Hopf algebra of quasisymmetric functions, where $\mathscr{P}(n)$ is the full subcategory of $\textbf{mod-}H_n(0)$ whose objects are direct sums of finitely many isomorphic copies of poset modules and $G_0(\mathscr{P}(n))$ is the Grothendieck group of $\mathscr{P}(n)$. We also demonstrate how (anti-)automorphism twists interact with these modules, the induction product and restrictions. Secondly, we investigate the (type 1) quasisymmetric power sum expansion of some quasi-analogues $Y_\alpha$ of Schur functions, where $\alpha$ is a composition. We show that they can be expressed as the sum of the $P$-partition generating functions of specific posets, which allows us to utilize the result established by Liu--Weselcouch. Additionally, we provide a new algorithm for obtaining these posets. Using these findings, for the dual immaculate function and the extended Schur function, we express the coefficients appearing in the quasisymmetric power sum expansions in terms of border strip tableaux.