“…We restrict our proofs to the "cubic domain case" (see the next section), where the interesting boundary condition (Dirichlet) is imposed on two opposite sides, and periodicity in the other two directions. This choice, introduced in reference [5] and used in a series of other papers (see for instance [4,6,10,11]), is convenient in order to work with a flat boundary and, at the same time, with a bounded domain. The main reason is that, in proving the regularity theorem for p > 2 (see Theorem 2.1), we apply the difference quotients method: we appeal to translations parallel to the flat boundary, and then retrieve the normal derivatives from the equations.…”