Maximum Estrada index of bicyclic graphs

Let GG be a simple graph of order nn, let λ1(G),λ2(G),…,λn(G)λ1(G),λ2(G),…,λn(G) be the eigenvalues of the adjacency matrix of GG. The Estrada index of GG is defined as EE(G)=∑i=1neλi(G). In this paper we determine the unique graph with maximum Estrada index among bicyclic graphs with fixed order.