Given a connected graph $R$ on $r$ vertices and a rooted graph $H,$ let $R\{ H\}$ be the graph obtained from $r$ copies of $H$ and the graph $R$ by identifying the root of the $i-th$ copy of $H$ with the $i-th$ vertex of $R$. Let $0\leq\alpha\leq1,$ and let \[ A_{ \alpha}(G)=\alpha D(G)+(1-\alpha)A(G) \] where $D(G)$ and $A(G)$ are the diagonal matrix of the vertex degrees of $G$ and the adjacency matrix of $G$, respectively. A basic result on the $A_{ \alpha}-$ spectrum of $R\{ H\}$ is obtained. This result is used to prove that if $H=B_{ k}$ is a generalized Bethe tree on $k$ levels, then the eigenvalues of $A_{ \alpha}(R\{ B_{ k}\})$ are the eigenvalues of symmetric tridiagonal matrices of order not exceeding $k$; additionally, the multiplicity of each eigenvalue is determined. Finally, applications to a unicyclic graph are given, including an upper bound on the $\alpha-$ spectral radius in terms of the largest vertex degree and the largest height of the trees obtained by removing the edges of the unique cycle in the graph.
Categories: math.CO