The maximum spectral radius of planner graphs without the joint of K2 and a linear forest

Abstract

Given a graph $F$, let $SPEX_P(n,F)$ be the set of graphs with the maximum spectral radius among all $F$-free $n$-vertex planner graph. In 2017, Tait and Tobin proved that for sufficiently $n$, $K_2+P_{n-2}$ is the unique graph with the maximum spectral radius over all $n$-vertex planner graphs. In this paper, focusing on $SPEX_P(n,K_2+H)$ in which $H$ is a linear forest, we prove that $SPEX_P(n,K_2+H)=\{2K_1+C_{n-2}\}$ when $H\in \{pK_2,P_3,I_q\}$ $(p\geq1, q\geq 3)$, where $K_n$, $P_n$, $I_n$ are complete graph, path and empty graph of order $n$, respectively. When $H$ contains a $P_4$, we prove that $2K_1+C_{n-2}\notin SPEX_P(n,K_2+H)$ and also provide a structural characterization of graphs in $SPEX_P(n,K_2+H)$.