First-Derivative Chromatic Symmetric Reconstruction For Proper Trees

Abstract

Let $T$ be a tree. Stanley asked whether the chromatic symmetric function $X_T$ determines $T$ up to isomorphism. We approach this open problem by regarding $X_T$ as a polynomial in the power-sum symmetric functions $p_1, p_2, \dots$ and studying the invariant $Φ_T = (\partial X_T/\partial p_1)|_{p_1 = 0}$. We prove that $Φ_T$ distinguishes every proper tree whose weighted skeleton, the tree obtained from $T$ by weighted contraction of all leaf edges, has distinct weights at non-leaf vertices. We prove further an equivalent formulation of Stanley's question obtained by attaching a fixed positive number of leaves to every vertex of a tree. Finally, we count spanning forests with at most $t$ edges, grouping them by the sizes of their connected components. We prove that these counts cannot distinguish all trees on $k \ge 4$ vertices unless $t \ge \lfloor k/2 \rfloor$.