The chromatic symmetric function of a graph centred at a vertex

We discover new linear relations between the chromatic symmetric functions of certain sequences of graphs and apply these relations to find new families of e-positive unit interval graphs. Motivated by the results of Gebhard and Sagan, we revisit their ideas and reinterpret their equivalence relation in terms of a new quotient algebra of NCSym. We investigate the projection of the chromatic symmetric function YG in noncommuting variables in this quotient algebra, which defines yG:v, the chromatic symmetric function of a graph G centred at a vertex v. We then apply our methods to yG:v and find new families of unit interval graphs that are (e)-positive, a stronger condition than classical e-positivity, thus confirming new cases of the (3 + 1)-free conjecture of Stanley and Stembridge. In our study of yG:v, we also describe methods of constructing new e-positive graphs from given (e)-positive graphs and classify the (e)-positivity of trees and cut vertices. We moreover construct a related quotient algebra of NCQSym to prove theorems relating the coefficients of yG:v to acyclic orientations of graphs, including a noncommutative refinement of Stanley’s sink theorem.

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