An improved bound for the strong clique index of graphs

Abstract

For a graph $G$ with line graph $L(G)$, $χ(L(G)^2)$ and $ω(L(G)^2)$ are called the \emph{strong chromatic index} and \emph{strong clique index} of $G$, respectively. A well-known conjecture of Erdős and Nešetřil (1985) posits that $χ(L(G)^2)\le \frac{5}{4}Δ(G)^2$. Related to that, Faudree, Gyárfás, Schelp and Tuza (1990) conjectured that $ω(L(G)^2) \le \frac{5}{4}Δ(G)^2$. We show that $ω(L(G)^2) \le \frac{2607}{1987}Δ(G)^2 < \frac{21}{16}Δ(G)^2$ improving the upper bound $\frac{4}{3}Δ(G)^2$ of Faron and Postle. Indeed, we make progress towards a stronger conjecture of Faron and Postle in terms of Ore-degree. For positive integers $Δ$ and $t$, let $h_t(Δ)$ denote the smallest integer such that any graph $G$ with size at least $h_t(Δ)$ and maximum degree $Δ(G)\le Δ$, contains two edges with distance at least $t$. An old problem of Erdős and Nešetřil (1986) concerns estimating the quantity $h_t(Δ)$ and can be thought of as the edge-version of the degree-diameter problem. Chung, Gyárfás, Tuza and Trotter established the sharp inequality $h_2(Δ)\le \frac{5}{4}Δ^2+1$. We disprove two conjectures of Cambie, Cames van Batenburg, Joannis de Verclos and Kang concerning the next open case $h_3(Δ)$.