A Combinatorial Perspective on the Noncommutative Symmetric Functions

TL;DR

This paper provides a concrete, self-contained exposition of noncommutative symmetric functions, offering explicit bases in noncommuting variables and exploring their change-of-basis interpretations without quasideterminants.

Contribution

It introduces explicit constructions of bases for noncommutative symmetric functions and interprets change-of-basis formulas through matrix products and combinatorial statistics.

Findings

Explicit bases expressed in noncommuting variables

Change-of-basis interpreted via matrix products

Connections to combinatorial statistics on brick tabloids

Abstract

The noncommutative symmetric functions $\textbf{NSym}$ were first defined abstractly by Gelfand et al. in 1995 as the free associative algebra generated by noncommuting indeterminants $\{\boldsymbol{e}_n\}_{n\in \mathbb{N}}$ that were taken as a noncommutative analogue of the elementary symmetric functions. The resulting space was thus a variation on the traditional symmetric functions $\Lambda$. Giving noncommutative analogues of generating function relations for other bases of $\Lambda$ allowed Gelfand et al. to define additional bases of $\textbf{NSym}$ and then determine change-of-basis formulas using quasideterminants. In this paper, we aim for a self-contained exposition that expresses these bases concretely as functions in infinitely many noncommuting variables and avoids quasideterminants. Additionally, we look at the noncommutative analogues of two different interpretations of change-of-basis in $\Lambda$: both as a product of a minimal number of matrices, mimicking Macdonald's exposition of $\Lambda$ in Symmetric Functions and Hall Polynomials, and as statistics on brick tabloids, as in work by E\u{g}ecio\u{g}lu and Remmel, 1990.