“…Indeed, it is shown in [26] that the small data solution u(t, x) to (1.3) decays like O(t −1/2 (log t) −1/2 ) in L ∞ (R x ) as t → +∞ if Im λ < 0. This gain of additional logarithmic time decay should be interpreted as another kind of longrange effect (see also [1], [2], [3], [4], [8], [9], [10], [11], [12], [13], [14], [16], [17], [18], [21], [24], [25], and so on). Time decay in L 2 -norm is also investigated by several authors.…”