The Estrada index of a simple connected graph GG of order nn is defined as EE(G)=∑ni=1eλiEE(G)=∑i=1neλi, where λ1,λ2,…,λnλ1,λ2,…,λn are the eigenvalues of the adjacency matrix of GG. In this paper, we characterize all tetracyclic graphs of order nn with maximal Estrada index.
