Connectivity and eigenvalues of graphs with given girth or clique number

Let $κ'(G)$, $κ(G)$, $μ_{n-1}(G)$ and $μ_1(G)$ denote the edge-connectivity, vertex-connectivity, the algebraic connectivity and the Laplacian spectral radius of $G$, respectively. In this paper, we prove that for integers $k\geq 2$ and $r\geq 2$, and any simple graph $G$ of order $n$ with minimum degree $δ\geq k$, girth $g\geq 3$ and clique number $ω(G)\leq r$, the edge-connectivity $κ'(G)\geq k$ if $μ_{n-1}(G) \geq \frac{(k-1)n}{N(δ,g)(n-N(δ,g))}$ or if $μ_{n-1}(G) \geq \frac{(k-1)n}{φ(δ,r)(n-φ(δ,r))}$, where $N(δ,g)$ is the Moore bound on the smallest possible number of vertices such that there exists a $δ$-regular simple graph with girth $g$, and $φ(δ,r) = \max\{δ+1,\lfloor\frac{rδ}{r-1}\rfloor\}$. Analogue results involving $μ_{n-1}(G)$ and $\frac{μ_1(G)}{μ_{n-1}(G)}$ to characterize vertex-connectivity of graphs with fixed girth and clique number are also presented. Former results in [Linear Algebra Appl. 439 (2013) 3777--3784], [Linear Algebra Appl. 578 (2019) 411--424], [Linear Algebra Appl. 579 (2019) 72--88], [Appl. Math. Comput. 344-345 (2019) 141--149] and [Electronic J. Linear Algebra 34 (2018) 428--443] are improved or extended.

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