Quasisymmetric Power Sums

TL;DR

This paper introduces and explores two new quasisymmetric power sum bases, providing initial combinatorial insights and contrasting their complexity with symmetric function analogues.

Contribution

It presents the first detailed study of the two quasisymmetric power sum bases, moving beyond duality and offering a combinatorial perspective.

Findings

Two distinct quasisymmetric power sum bases are characterized.

The bases have complex combinatorial descriptions unlike symmetric counterparts.

The paper encourages combinatorial approaches over duality for these bases.

Abstract

In the 1995 paper entitled "Noncommutative symmetric functions," Gelfand, et. al. defined two noncommutative symmetric function analogues for the power sum basis of the symmetric functions, along with analogues for the elementary and the homogeneous bases. They did not consider the noncommutative symmetric power sum duals in the quasisymmetric functions, which have since been explored only in passing by Derksen and Malvenuto-Reutenauer. These two distinct quasisymmetric power sum bases are the topic of this paper. In contrast to the simplicity of the symmetric power sums, or the other well known bases of the quasisymmetric functions, the quasisymmetric power sums have a more complex combinatorial description. As a result, although symmetric function proofs often translate directly to quasisymmetric analogues, this is not the case for quasisymmetric power sums. Neither is there a model for working with the quasisymmetric power sums in the work of Gelfand, et. al., which relies heavily on quasi-determinants (which can only be exploited by duality for our purposes) and is not particularly combinatorial in nature. This paper therefore offers a first glimpse at working with these two relatively unstudied quasisymmetric bases, avoiding duality where possible to encourage a previously unexplored combinatorial understanding.