A rooted variant of Stanley's chromatic symmetric function

Abstract

Richard Stanley defined the chromatic symmetric function $X_G$ of a graph $G$ and asked whether there are non-isomorphic trees $T$ and $U$ with $X_T=X_U$. We study variants of the chromatic symmetric function for rooted graphs, where we require the root vertex to either use or avoid a specified color. We present combinatorial identities and recursions satisfied by these rooted chromatic polynomials, explain their relation to pointed chromatic functions and rooted $U$-polynomials, and prove three main theorems. First, for all non-empty connected graphs $G$, Stanley's polynomial $X_G(x_1,\dots ,x_N)$ is irreducible in $\mathbb{Q}[x_1,\dots ,x_N]$ for all large enough $N$. The same result holds for our rooted variant where the root node must avoid a specified color. We prove irreducibility by a novel combinatorial application of Eisenstein's Criterion. Second, we prove the rooted version of Stanley's Conjecture: two rooted trees are isomorphic as rooted graphs if and only if their rooted chromatic polynomials are equal. In fact, we prove that a one-variable specialization of the rooted chromatic polynomial (obtained by setting $x_0=x_1=q$, $x_2=x_3=1$, and $x_n=0$ for $n>3$) already distinguishes rooted trees. Third, we answer a question of Pawlowski by providing a combinatorial interpretation of the monomial expansion of pointed chromatic functions.

21 pages; v2: added a short algebraic proof to Theorem 2 (now Theorem 15), we also answer a question of Pawlowski about monomial expansions; v3: added additional one-variable specialization results, simplified main proofs