Bounds on the Estrada index of ISR ( 4 , 6 ) -fullerenes

Suppose G is a graph and λ 1 , λ 2 , … λ n are the eigenvalues of G . The Estrada index E E ( G ) of G is defined as the sum of the terms e λ i , 1 ≤ i ≤ n . In this work some upper and lower bounds for the Estrada index of ( 4 , 6 ) -fullerene graphs are presented.