“…Then, in the case of an unbounded domain (whole or exterior domain) we can observe that the acoustic waves redistribute their energy in the space and so one can exploit the dispersive properties of these waves to get the local decay of the acoustic energy and to recover compactness in time, see for example [5], [7], [8], [13]. In the case of a periodic domain we don't have a dispersion phenomenon but the waves interact with each other, so in the spirit of Schochet [28] and [29] one has to introduce an operator which describes the oscillations in time so that they can be included in the energy estimates, see [25], [26], [15], [22]. In this paper we will study the incompressible limit in a periodic domain for the system of quantum hydrodynamics (7) and, as explained above, the main issue is to control the time oscillations of the density fluctuation and of the momentum J ε .…”