A Combinatorial Perspective on the Noncommutative Symmetric Functions

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.