“…(n, v +c, ϕ)(x −ct, t) and, respectively, (n, v, ϕ)(−x, −t) are solutions, if (n, v, ϕ)(x, t) is a solution. We refer to [15,21,29] and the references therein for more information on the physical background of these equations. A particular solution is found for constant density n ≡ 1, vanishing potential ϕ ≡ 0, and constant velocity v ≡ v 0 ; here v 0 is arbitrary due to Galilean invariance.…”