“…On the other hand, abstract equations like (1.1) are also treated by Ikehata-Nishihara [12], Chill-Haraux [3], Radu-TodorovaYordanov [18] in the case when A 1 = A 2 = I and A 3 = nonnegative self-adjoint operator in a Hilbert space, and by Ikehata-Todorova-Yordanov [13] in the case when A 1 = I and A 2 = A 3 = nonnegative self-adjoint operator in a Hilbert space, in order to investigate the diffusion phenomenon of solutions. In addition to these works, recently in Lu-Reissig [16], IkehataNatsume [11], Charão-da Luz-Ikehata [2], D'Abbicco-Reissig [7] and D'Abbicco-Ebert [5,6] they studied optimal decay estimates of solutions for (1.1) in the case where A 1 = I , A 2 = (− ) θ (θ ∈ [0, 1]), and A 3 = − , which corresponds to the wave equations with structural damping. Quite recently, two papers due to Charão-da Luz-Ikehata [1,10] have been successively published based on a new method introduced in [2].…”