The Newton polytope and Lorentzian property of chromatic symmetric functions

Abstract: Chromatic symmetric functions are well-studied symmetric functions in algebraic combinatorics that generalize the chromatic polynomial and are related to Hessenberg varieties and diagonal harmonics. Motivated by the Stanley--Stembridge conjecture, we show that the allowable coloring weights for indifference graphs of Dyck paths are the lattice points of a permutahedron $\mathcal{P}_\lambda$, and we give a formula for the dominant weight $\lambda$. Furthermore, we conjecture that such chromatic symmetric functions are Lorentzian, a property introduced by Br\"and\'en and Huh as a bridge between discrete convex analysis and concavity properties in combinatorics, and we prove this conjecture for abelian Dyck paths. We extend our results on the Newton polytope to incomparability graphs of (3+1)-free posets, and we give a number of conjectures and results stemming from our work, including results on the complexity of computing the coefficients and relations with the $\zeta$ map from diagonal harmonics.