“…In fact, the smoothing effect is a natural property for the whole R n or non-trapping exterior domains as considered in the works of Constantin and Saut [14] or Burq, Gérard and Tzvetkov [10]. It has been considered previously in unbounded domains as powerful tool in order to achieve the exponential stability, see, for instance, the works [8] and [12]. On the other hand, for compact manifolds the smoothing effect has been introduced firstly by Aloui [1,2], by forcing it precisely by means of the stronger damping iαb(x)(−∆) 1 2 b(x) above mentioned where the the region ω such that b(x) > 0 satisfies the wellknown geometric control condition (GCG, in short).…”