A lift of chromatic symmetric functions to $\textsf{NSym}$

Abstract

If we consider previously introduced extensions of Stanley's chromatic symmetric function $X_{G}(x_1, x_2, \ldots)$ for a graph $G$ to elements in the algebra $\textsf{QSym}$ of quasisymmetric functions and in the algebra $\textsf{NCSym}$ of symmetric functions in noncommuting variables, this motivates our introduction of a lifting of $X_{G}$ to the dual of $\textsf{QSym}$, i.e., the algebra $\textsf{NSym}$ of noncommutative symmetric functions, as opposed to $\textsf{NCSym}$. For an unlabelled directed graph $D$, our extension of chromatic symmetric functions provides an element $\text{{X}}_{D}$ in $\textsf{NSym}$, in contrast to the analogue $Y_{G} \in \textsf{NCSym}$ of $X_{G}$ due to Gebhard and Sagan. Letting $G$ denote the undirected graph underlying $D$, our construction is such that the commutative image of $\text{{X}}_{D}$ is $ X_{G}$. This projection property is achieved by lifting Stanley's power sum expansion for chromatic symmetric functions, with the use of the $\Psi$-basis of $\textsf{NSym}$, so that the orderings of the entries of the indexing compositions are determined by the directed edges of $D$. We then construct generating sets for $\textsf{NSym}$ consisting of expressions of the form $\text{{X}}_{D}$, building on the work of Cho and van Willigenburg on chromatic generating sets for $\textsf{Sym}$.