LLT cumulants and graph coloring

TL;DR

This paper introduces LLT cumulants, a new family of quasi-symmetric functions, and explores their properties, combinatorial interpretations, and positivity results, providing evidence for their Schur-positivity and implications for Macdonald polynomials.

Contribution

The paper defines LLT cumulants, establishes their positivity properties, and connects them to Macdonald cumulants, advancing understanding of symmetric functions and graph colorings.

Findings

LLT cumulants have a positive expansion in terms of ribbon shapes.

Positivity results include monomial, fundamental quasisymmetric, and e-positivity.

Schur-positivity of LLT cumulants is conjectured and linked to Macdonald cumulants.

Abstract

The purpose of this note is to introduce a new family of quasi-symmetric functions called LLT cumulants and discuss its properties. We define LLT cumulants using the algebraic framework for conditional cumulants and we prove that the Macdonald cumulant has an explicit positive expansion in terms of LLT cumulants of ribbon shapes, generalizing the classical decomposition of Macdonald polynomials. We also find a natural combinatorial interpretation of the LLT cumulant of a given directed graph as a weighted generating function of colorings of its subgraphs. We use this graph theoretical framework to prove various positivity results. This includes monomial positivity, positivity in fundamental quasisymmetric functions and related positivity of the coefficients of Schur polynomials indexed by hook shapes. We also prove $e$-positivity for vertical-shape LLT cumulants, after the shift of variable $q \to q+1$, which refines a recent result of Alexandersson and Sulzgruber. All these results give evidence towards Schur-positivity of LLT cumulants, which we conjecture here. We prove that this conjecture implies Schur-positivity of Macdonald cumulants, and we give more evidence by proving the conjecture for LLT cumulants of melting lollipops that refines a recent result of Huh, Nam and Yoo.