Spectral extremal problem on the square of $\ell$-cycle

Abstract: Let $C_{\ell}$ be the cycle of order ${\ell}$. The square of $C_{\ell}$, denoted by $C_{\ell}^2$, is obtained by joining all pairs of vertices with distance no more than two in $C_{\ell}$. A graph is called $F$-free if it does not contain $F$ as a subgraph. Denote by $ex(n,F)$ and $spex(n,F)$ the maximum size and spectral radius over all $n$-vertex $F$-free graphs, respectively. The well-known Tur\'{a}n problem asks for the $ex(n,F)$, and Nikiforov in 2010 proposed a spectral counterpart, known as Brualdi-Solheid-Tur\'{a}n type problem, focusing on determining $spex(n,F)$. In this paper, we consider a Tur\'{a}n problem on $ex(n,C_{\ell}^2)$ and a Brualdi--Solheid--Tur\'{a}n type problem on $spex(n,C_{\ell}^2)$. We give a sharp bound of $ex(n,C_{\ell}^2)$ and $spex(n,C_{\ell}^2)$ for sufficiently large $n$, respectively. Moreover, in both results, we characterize the corresponding extremal graphs for any integer $\ell\geq 6$ that is not divisible by $3$.