Essential connectivity and spectral radius of graphs

A graph is trivial if it contains one vertex and no edges. The essential connectivity ^ of G is defined to be the minimum number of vertices of G whose removal produces a disconnected graph with at least two non-trivial components. Let Aₙ^', be the set of graphs of order n with minimum degree and essential connectivity '. In this paper, we determine the graphs attaining the maximum spectral radii among all graphs in Aₙ^', and characterize the corresponding extremal graphs. In addition, we also determine the digraphs which achieve the maximum spectral radii among all strongly connected digraphs with given essential connectivity and give the exact values of the spectral radii of these digraphs.