Extremal spectral results of planar graphs without $C_{l, l}$ or $\mathrm{Theta}$ graph

Abstract

Let $\mathcal {F}$ be a given family of graphs. A graph $G$ is $\mathcal {F}$-free if it does not contain any member of $\mathcal {F}$ as a subgraph. Let $C_{l, l}$ be a graph obtained from $2C_l$ such that the two cycles share a common vertex, where $l\geqslant3 $. A $\mathrm{Theta}$ graph is obtained from a cycle $C_k$ by adding an additional edge between two non-consecutive vertices on $C_k$, where $k\geqslant 4$. Let $\Theta_k$ be the set of $\mathrm{Theta}$ graphs on $k$ vertices, where $k \geqslant 4$. For sufficiently large $n $, the unique extremal planar graph with the maximum spectral radius among $C_{l, l}$-free planar graphs on $n$ vertices and among $\Theta_k$-free planar graphs on $n$ vertices are characterized respectively, where $l \geqslant 3$ and $k \geqslant 4$.