“…In these articles, the existence of global attractors has been proved when the coefficient α(·) of the weak damping term is strictly positive (see [12]) or, in addition to (1.3), is positive (see [13]) almost everywhere in R n . However, in the case when α(·) vanishes in a set of positive measure, the existence of the global attractor for (1.1) with β ≡ 0 remained as an open question (see [12,Remark 1.2]). On the other hand, in the case when α ≡ 0 and even β ≡ 1, the semigroup {S (t)} t≥0 generated by (1.1)-(1.2) does not possess a global attractor in H 2 (R n ) × L 2 (R n ).…”