“…In recent years, various types of wave equations with linear or nonlinear damping and sources have been solved by using the Galerkin approximation (see, for instance, [2,12,14,15,17]). Based on a priori estimates, weak convergence, and compactness techniques, and via the construction of a suitable Lyapunov functional, the existence, regularity, blow-up, and exponential decay estimates of solutions for such typical wave equations have been proved in [2,12,17].…”