Some extremal problems on [formula omitted]-spectral radius of graphs with given size

Abstract

Nikiforov defined the Aα-matrix of a graph G as Aα(G)=αD(G)+(1−α)A(G), where α∈[0,1], D(G) and A(G) are the diagonal matrix of degrees and the adjacency matrix respectively. The largest eigenvalue of Aα(G) is called the Aα-spectral radius of G, denoted by ρα(G). In this paper, we first give an upper bound on ρα(G) of a connected graph G with fixed size m≥3k and maximum degree Δ≤m−k, where k is a positive integer. For two connected graphs G1 and G2 with size m≥4, employing this upper bound, we prove that ρα(G1)>ρα(G2) if Δ(G1)>Δ(G2) and Δ(G1)≥2m3+1. As an application, we determine the graph with the maximal Aα-spectral radius among all graphs with fixed size and girth. Our theorems generalize the recent results for the signless Laplacian spectral radius of a graph.