Hamid Reza Bamdad - Department of Mathematics, Payame Noor University (PNU), Tehran, Iran

Let G be an n-vertex graph. If λ۱, λ۲, . . . , λn are the eigenvalues of its adjacencymatrix, the Estrada index of G defined as EE(G) =Σni=۱ eλi . In this paper by use ofthe linier algebraic tools we obtain a lower bound for EE(G) and show that it is bestpossible.

Estrada index, Adjacency matrix, Complete graph, Spectrom of graphs