Sharp bounds on the $A_{\alpha}$-index of graphs in terms of the independence number

Abstract

Given a graph $G$, the adjacency matrix and degree diagonal matrix of $G$ are denoted by $A(G)$ and $D(G)$, respectively. In 2017, Nikiforov \cite{0007} proposed the $A_{\alpha}$-matrix: $A_{\alpha}(G)=\alpha D(G)+(1-\alpha)A(G),$ where $\alpha\in [0, 1]$. The largest eigenvalue of this novel matrix is called the $A_\alpha$-index of $G$. In this paper, we characterize the graphs with minimum $A_\alpha$-index among $n$-vertex graphs with independence number $i$ for $\alpha\in[0,1)$, where $i=1,\lfloor\frac{n}{2}\rfloor,\lceil\frac{n}{2}\rceil,{\lfloor\frac{n}{2}\rfloor+1},n-3,n-2,n-1,$ whereas for $i=2$ we consider the same problem for $\alpha\in [0,\frac{3}{4}{]}.$ Furthermore, we determine the unique graph (resp. tree) on $n$ vertices with given independence number having the maximum $A_\alpha$-index with $\alpha\in[0,1)$, whereas for the $n$-vertex bipartite graphs with given independence number, we characterize the unique graph having the maximum $A_\alpha$-index with $\alpha\in[\frac{1}{2},1).$