Estimating the Estrada Index of a Graph

Let G be an n-vertex graph. If λ1, λ2, . . . , λn are the eigenvalues of its adjacency matrix, the Estrada index of G defined as EE(G) = ∑(n, i=1) e^λi . In this paper by use of the linier algebraic tools we obtain a lower bound for EE(G) and show that it is best possible.