Sharp control time for viscoelastic bodys

Abstract: It is now well understood that equations of viscoelasticity can be seen as perturbation of wave type equations. This observation can be exploited in several different ways and it turns out that it is a usefull tool when studying controllability. Here we compare a viscoelastic system which fills a surface of a solid region (the string case has already been studied) with its memoryless counterpart (which is a generalized telegraph equation) in order to prove exact controllability of the viscoelastic body at precisely the same times at which the telegraph equation is controllable. The comparison is done using a moment method approach to controllability and we prove, using the perturbations theorems of Paley-Wiener and Bari, that a new sequence derived from the viscoelastic system is a Riesz sequence, a fact that implies controllability of the viscoelastic system. The results so obtained generalize existing controllability results and furthermore show that the ``sharp'' control time for the telegraph equation and the viscoelastic system coincide.