The twinning operation on graphs does not always preserve $e$-positivity

TL;DR

This paper investigates the preservation of $e$-positivity under the twinning operation on graphs, providing counterexamples to a conjecture and exploring the limits of $e$-positivity in related graph classes.

Contribution

It disproves Foley, Hoàng, and Merkel's conjecture that twinning preserves $e$-positivity for all $e$-positive graphs, and introduces new insights into the behavior of $e$-positivity in complex graph operations.

Findings

Counterexamples show twinning does not always preserve $e$-positivity.

Tadpole graphs are established as $e$-positive.

Twin and clan graphs may lose $s$-positivity after twinning.

Abstract

Motivated by Stanley's $\mathbf{(3+1)}$-free conjecture on chromatic symmetric functions, Foley, Ho\`{a}ng and Merkel introduced the concept of strong $e$-positivity and conjectured that a graph is strongly $e$-positive if and only if it is (claw, net)-free. In order to study strongly $e$-positive graphs, they further introduced the twinning operation on a graph $G$ with respect to a vertex $v$, which adds a vertex $v'$ to $G$ such that $v$ and $v'$ are adjacent and any other vertex is adjacent to both of them or neither of them. Foley, Ho\`{a}ng and Merkel conjectured that if $G$ is $e$-positive, then so is the resulting twin graph $G_v$ for any vertex $v$. Based on the theory of chromatic symmetric functions in non-commuting variables developed by Gebhard and Sagan, we establish the $e$-positivity of a class of graphs called tadpole graphs. By considering the twinning operation on a subclass of these graphs with respect to certain vertices we disprove the latter conjecture of Foley, Ho\`{a}ng and Merkel. We further show that if $G$ is $e$-positive, the twin graph $G_v$ and more generally the clan graphs $G^{(k)}_v$ ($k \ge 1$) may not even be $s$-positive, where $G^{(k)}_v$ is obtained from $G$ by applying $k$ twinning operations to $v$.