“…This gain of additional logarithmic time decay should be interpreted as another kind of long-range effect. Among several extensions of this result (see e.g., [3], [9], [11], [12], [13] etc. and the references cited therein), let us focus on the following two cases: (i) the case where the nonlinearity depends also on ∂ x u, and (ii) the case of systems.…”