Abstract

Let G be a graph on n vertices and lambda(1) >= lambda(2) >= ... lambda(n), its eigenvalues. The Estrada index of G is defined as EE(G) = Sigma(n)(i=1) e(lambda i). In this work, we use an increasing sequence converging to the lambda(1) to obtain an increasing sequence of lower bounds for EE(G). In addition, we generalize this succession for the Estrada index of an arbitrary symmetric nonnegative matrix. (C) 2019 Elsevier Inc. All rights reserved.