“…To solve (1.2), even locally in time, one needs to work in Σ, and not only H 1 ; symmetry is needed on physical and frequency sides, unless V is sublinear [9]. Local existence in Σ then follows from the dispersive estimates in [20,21]; one can work as in the case V ≡ 0 (where it is possible to work in H 1 (R d ) only). Instead of considering only (u,∇u) as the unknown function, one can consider (u,∇u,xu).…”