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

Abstract

Motivated by Stanley's $(3+1)$-free conjecture on chromatic symmetric functions, Foley, Hoàng 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 $v^{\prime }$ to $G$ such that $v$ and $v^{\prime }$ are adjacent and any other vertex is adjacent to both of them or neither of them. Foley, Hoàng 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, Hoàng and Merkel. We further show that if $G$ is $e$-positive, the twin graph $G_v$ and more generally the clan graphs $G_v^{(k)}$ ($k\ge 1$) may not even be $s$-positive, where $G_v^{(k)}$ is obtained from $G$ by applying $k$ twinning operations to $v$.

20 pages, 17 figures