Sequence of Bounds for Spectral Radius and Energy of Digraph.

The graph spectra analyze the structure of the graph using eigenspectra. The spectral graph theory deals with the investigation of graphs in terms of the eigenspectrum. In this paper, the sequence of lower bounds for the spectral radius of digraph D having at least one doubly adjacent vertex in terms of indegree is proposed. Particularly, it is exhibited that ρ (D) ≥ α j = ∑ p = 1 m (χ j + 1 (p)) 2 ∑ p = 1 m (χ j (p)) 2 , such that equality is attained iff D = G ↔ + { DE ∉ Cycle}, where each component of associated graph is a k-regular or (k 1 , k 2) semiregular bipartite. By utilizing the sequence of lower bounds of the spectral radius of D , the sequence of upper bounds of energy of D , where the sequence decreases when e U ≤ α j and increases when e U > α j , are also proposed. All of the obtained inequalities are elaborated using examples. We also discuss the monotonicity of these sequences. [ABSTRACT FROM AUTHOR]