Global Attractors for Damped Semilinear Wave Equations

The existence of a global attractor in the natural energy space is proved for the semilinear wave equation utt + βut − ∆u + f(u) = 0 on a bounded domain Ω ⊂ R with Dirichlet boundary conditions. The nonlinear term f is supposed to satisfy an exponential growth condition for n = 2, and for n ≥ 3 the growth condition |f(u)| ≤ c0(|u|γ + 1), where 1 ≤ γ ≤ n n−2 . No Lipschitz condition on f is assumed, leading to presumed nonuniqueness of solutions with given initial data. The asymptotic compactness of the corresponding generalized semiflow is proved using an auxiliary functional. The system is shown to possess Kneser’s property, which implies the connectedness of the attractor. In the case n ≥ 3 and γ > n n−2 the existence of a global attractor is proved under the (unproved) assumption that every weak solution satisfies the energy equation. Dedicated to M.I. Vishik on the occasion of his 80 birthday