Refinement of Hikita's $e$-positivity theorem via Abreu--Nigro's $g$-functions and restricted modular law

Abstract

We study the symmetric functions \( g_{\mm,k}(x;q) \), introduced by Abreu and Nigro for a Hessenberg function \( \mm \) and a positive integer \( k \), which refine the chromatic symmetric function. Building on Hikita's recent breakthrough on the Stanley--Stembridge conjecture, we prove the \( e \)-positivity of \( g_{\mm,k}(x;1) \), refining Hikita's result. We also provide a Schur expansion of the sum \( \sum_{k=1}^n e_k(x) g_{\mm,n-k}(x;q) \) in terms of \( P \)-tableaux with 1 in the upper-left corner. We introduce a restricted version of the modular law as our main tool. Then, we show that any function satisfying the restricted modular law is determined by its values on disjoint unions of path graphs.