A combinatorial expansion of vertical-strip LLT polynomials in the basis of elementary symmetric functions

Abstract

We give a new characterization of the vertical-strip LLT polynomials ${\mathrm{LLT}}_P(x;q)$ as the unique family of symmetric functions that satisfy certain combinatorial relations. This characterization is then used to prove an explicit combinatorial expansion of vertical-strip LLT polynomials in terms of elementary symmetric functions. Such formulas were conjectured independently by A. Garsia et al. and the first named author, and are governed by the combinatorics of orientations of unit-interval graphs. The obtained expansion is manifestly positive if $q$ is replaced by $q+1$, thus recovering a recent result of M. D'Adderio. Our results are based on linear relations among LLT polynomials that arise in the work of D'Adderio, and of E. Carlsson and A. Mellit. To some extent these relations are given new bijective proofs using colorings of unit-interval graphs. As a bonus we obtain a new characterization of chromatic quasisymmetric functions of unit-interval graphs.

49 pages. This version has updated .bib, and some improvements in section 6

References in corpus (3)

• A directed graph generalization of chromatic quasisymmetric functions
• A symmetric function of increasing forests
• $e$-Positivity Results and Conjectures

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• Chromatic symmetric functions of Dyck paths and q-rook theory (extended abstract)
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• LLT cumulants and graph coloring